Abstract: At present, distinguishing overlapped leaves is the difficulty of measuring LAI（leaf area index）. In this paper, a new method for the detection of LAI of overlapped leaves was proposed. The citrus leaves were put under the direct parallel light and the photoelectric sensor was used to measure the transmitted light energy. The output voltage signal of the sensor was sent into the computer through the data acquisition card. Then, the amount of overlapped layers was determined automatically by LabVIEW software programming. The amount of overlapped layers multiplied by the area of the sensor receiver was the area of the leaves. The total area of the leaves was the cumulative leaf area in the whole testing process and the LAI could be calculated. According to the experimental results, near-infrared was chosen as the photosource and a near-infrared photoelectric sensor was designed. The short circuit current I1 and I2 had a good linear relationship with the preamplifiers output voltage U1 and the main output voltage U2. The significant level of R2 was p<0.05. The data acquisition experimental results showed that the absolute value of the maximum relative error between the value of LAI calculated by this platform and the real value of LAI obtained by grid method was 14.8%, in the mode of manual scanning. Moreover, the LAI with distant leaves was tested in preliminary stage showing a power function with the leaf distance significantly.