The effect of irradiation angle on detecting apple sugar by near infrared spectroscopy is discussed, and the best irradiative way of light source is obtained. An apple is selected as the object and sugar content of the apple is measured. The apple sample is irradiated in the middle of two lights of 100 W with three irradiation angles (30°, 45°, 60°). The spectra are collected using diffuse transmittance and the partial least squares regression model (PSL) is established. When the light angle is 45°, the precision of the model is the highest. The correlation coefficient of the calibration set (R_c) is 0.91, root mean square error of the calibration set (x_RMSEC) is 0.57, the correlation coefficient of the prediction set (R_p) is 0.84 and the root mean square error of the prediction set (x_RMSEC) is 0.73. The results show that the irradiation angle has certain effect on the model accuracy, and the study provides a basis for selecting the best irradiation angle in the building of actual models.