In this paper, the authors derived the relationship between the travel time difference of sPn and Pn and the local earthquake focal depth. In these equations, the travel time difference of sPn and Pn is not related to the epicentral distance, only depended on the regional crustal model and the focal depth. According to equation, the authors provided a simple and accurate method to determine local earthquake focal depth by using the travel time difference between phase sPn and Pn. This method has been used to determine the focal depths of two earthquakes of MS6.1 and MS5.6 occurred at the junction of Panzhihua and Huili, Sichuan Province on Aug 30 and 31, 2008. The results have been compared with those from other sources such as China Earthquake Network Center, showing that the results are accurate and reliable.