Based on P3 approximate spatially-resolved diffuse reflectance theory for semi-infinite biological tissue, the simplified expression SP3 function for highly absorbing medium close to source is studied. The research result shows that function SP3 can better describe the spatially-resolved diffuse reflectance close to source for higher absorption; the inversion of the optical parameters of highly absorbing medium with function SP3 shows the existence of extreme values of the curves fitted with diffuse reflectance data for different start distances, and the inversion results near the extreme values are the most closest to the optical parameters of real tissues.