This paper modifies an analytical algorithm originally developed for electron dose calculations to evaluate the off-axis dose distribution of rectangle proton beam. This spatial distribution could be described by Fermi-Eyges theory since a proton undergoes small-angle scattering when it passes through medium. Predictions of the algorithm for relative off-axis dose distribution by a 6 cm *6 cm initial monoenergetic proton beam are compared with the results from the published Monte Carlo simulations. The excellent level of agreement between the results of these two methods of dose calculation (< 2%) demonstrates that the off-axis dose distribution from rectangle proton beam may be computed with high accuracy using this algorithm. The results also prompts the necessity to consider the off-axis distribution when the proton is applied to clinical radiotherapy since the penumbra is significant at the distal of its range (about 0.6 cm at the Bragg-peak depth). This paper modifies an analytical algorithm originally developed for electron dose calculations to evaluate the off-axis dose distribution of rectangular proton beam. This spatial distribution could be described by Fermi-Eyges theory since a proton undergoes small-angle scattering when it passes through medium. Predictions of the algorithm for relative off-axis dose distribution by a 6 cm * 6 cm initial monoenergetic proton beam are compared with the results from the published Monte Carlo simulations. The excellent level of agreement between the results of these two methods of dose calculation ( <2%) demonstrates that the off-axis dose distribution from rectangle proton beam may be computed with high accuracy using this algorithm. The results also prompts the necessity to consider the off-axis distribution when the proton is applied to clinical radiotherapy since the penumbra is significant at the distal of its range (about 0.6 cm at the Bragg-peak depth).