A new mathematical model for detecting the contents of chlorophyll, nitrogen and water of plants at the same time was established. Combined with the theory of spectroscopy, a portable meter for detecting the contents of chlorophyll, nitrogen and water in crop leaves was developed, which could avoid the error caused by different measuring time and different measuring location on the leaves when the three measuring parameters were accomplished by different instruments. The measurement result could give a better suggestion to precision fertilization and irrigation. The developed meter was composed of NIR (near infrared) LED (light emitting diode), interference filter, photodetector and microprocessor system.