In order to construct a fast measurement system for leaf area index (LAI) based on near-infrared reflection, an innovative way was explored to accurately detect the LAI of overlapping leaves. Research methods were concluded as follows: overlapping leaves were put under the direct parallel light generated by the near-infrared laser, and the photoelectric sensors were used to measure the reflected light energy. The output voltage signal of the sensors was sent into the computer through the data acquisition card. The amount of overlapping layers was determined by software programming. The amount of overlapping layers was multiplied by the area of direct beam’s light spot was the area of the leaves. The total area of the leaves was the cumulative leaf area in the whole testing process and LAI could be calculated. Based on the result of spectral reflection test, near-infrared was used as direct light source to demarcate the system to get the index fitting model between overlapping leaves’number and measuring voltage, which is notable in 0.05 statistic level. The result of data collection experiment shows that under the method in manual scanning and timing collection, the value of LAI was calculated by the system with 11.01% of the error compared with the true value of LAI calculated by square method.