Leaf area index (LAI) of fruit trees provides significant references for precision spraying. However, measuring LAI from the vertical direction conventionally does not conform to the actual demand of lateral precision spraying. In this paper, using measuring system of citrus LAI with near-infrared transmittance technique, LAI of citrus trees was measured from the horizontal direction, the total leaf area was calculated, and then the real leaf area was obtained by direct method as standard values to measure the error. The experimental result was that the average absolute value of the absolute errors were 0.45 and 0.40 respectively, and average absolute value of the relative errors were 17.37% and 14.91% respectively when measuring in east-west direction and south-north direction. Besides the error of measuring the total leaf area was not much different with the error of measuring from vertical direction by using the system or WinSCANOPY Canopy analysis system. It shows that the accuracy of measuring LAI or total leaf area from horizontal direction meets the requirement of the experiment.