Maize is one of the main grain crops in Northeast China. Droughts are often accompanied by low temperatures during seedling stage in the region, which often harmfully affect the establishment of the seedling. Drip irrigation under plastic film mulch is an alternative that has been applied to maize production in Northeast China in recent years for supplying complementary irrigation and increasing accumulated temperature. For better understanding of the effects of mulched drip irrigation on water and heat conditions in field and maize yield in sub-humid regions, field experiments were carried out in the sub-humid environment of Heilongjiang Province, China, during the growing seasons of maize in 2011, 2012 and 2013. In the experiments, three treatments, including mulched drip irrigation, non-mulched drip irrigation and surface irrigation were tested. The daily soil temperature was recorded manually at 08:00, 14:00, and 18:00 at 5 cm intervals within the 5~25 cm profile by geothermometers in 2012 and 2013. In addition, high-revolution soil water-heat-salt automatic systems (ECH2O) were used to continuously monitor soil water content and temperature at fixed locations of the mulched and non-mulched drip irrigation treatments within the 20~80 cm profile in 2013. Micro-lysimeters (MLS) with inner diameter of 10 cm and height of 15 cm were fabricated to measure evaporation from the soil surface during the growing season of 2013. Additionally, air temperature and humidity in canopy were measured using a Kestrel 4 000 pocket weather station during selected typical days. Plant height, leaf area index (LAI), aboveground dry matter, and nitrogen uptake were also measured during the seedling, jointing, tasseling, filling, and physiological maturity stages of maize. The results indicated that mulched drip irrigation increased the daily temperature at 5~25 cm soil depth by 2.3℃ compared with that of the non-mulched treatments. Accordingly, the accumulated soil temperature for the mulched drip irrigation treatment was about 87℃ higher than that for the non-mulched treatments during the seedling stage, which led to the highest increment of accumulated soil temperature during the whole growing season (115~150℃). Furthermore, the daily evaporation rate from the soil surface for the mulched drip irrigation treatment (0.7 mm/d) was 53% lower than that for the non-mulched drip irrigation treatment (1.5 mm/d), resulting in a higher water content in topsoil layer during the early growing season. Compared with the non-mulched drip irrigation treatment, the mulched drip irrigation produced a higher air temperature and lower air humidity in canopy at sunny days of the jointing stage, implying a greater transpiration rate for the mulched drip irrigation treatment. In terms of crop growth, the growth speed of plant height, LAI, aboveground dry matter and nitrogen uptake with the growing degree days (GDD) for the mulched drip irrigation treatment was obviously greater than those for the non-mulched drip irrigation and surface irrigation treatments during the early growing stage. At the physiological maturity stage, the aboveground dry matter averaged over the three growing seasons for the mulched drip irrigation treatment was 14% and 23% greater than that for the non-mulched drip irrigation and surface irrigation treatments, respectively. Accordingly, the nitrogen uptake for the mulched drip irrigation treatment was 16% and 28% greater than those for the non-mulched drip irrigation and surface irrigation treatments, respectively. The three-year average grain yield and water use efficiency (WUE) for the mulched drip irrigation treatment were 11% and 8% greater than those for the non-mulched drip irrigation treatment, and 21% and 18% greater than those for surface irrigation treatment, respectively. In conclusion, mulched drip irrigation produced a more favorable water-heat environment in field which promoted crop growth and improved maize yield and WUE.