To measure the focal depth by using the travel time difference between sPn and Pn waves and further improve the measurement accuracy, the relationship between the travel time difference of sPn and Pn waves in multilayer medium and the focal depth was deduced, and it was found that the travel time difference is not related to the epicenter distance, but only related to the focal depth and the regional crustal velocity model. The slope of the travel time difference curve remains unchanged with the source in the same layer, while the slope of the curve is different with the source in different layers, showing as a piecewise straight line. The more uneven the crustal velocity structure is in the longitudinal direction, the greater the focal depth error calculated by sPn-Pn travel time difference is, and the greater the slope difference of each segment of the travel time curve is. The single-layer crustal velocity model of sPn and Pn waves with the same propagation path but different wave velocities in the crust was discussed, and it was found that the measured focal depth increases with the increase of wave velocity in single-layer medium. For the same layered structure of crust in the same area, the measured focal depth decreases with the increase of Poisson's ratio. Based on the multi-layer average crustal model of five typical blocks in China provided by previous studies, a quick look-up table for the formula to calculate the focal depth by using the sPn-Pn travel time difference was presented.