A precise and rapid analytical algorithm has been developed for calculating proton energy deposition in media. The results covering the radiotherapy energy range (50—250MeV) from this algorithm and data from Monte Carlo code are given. Comparison between them shows that 1) The Nonelastic nuclear interaction have severe influence on proton energy deposition, especially when the initial energy of proton beams is high. 2) The algorithm is 100 times faster than Monte Carlo methods. It satisfies the requirement of precision and speed. The algorithm shows its great potentiality in proton radiotherapy.