The environmental radius into the Fujita formula by consideration of environmental pressure is introduced and applied in improving typhoon 9711 forecasting. The results show that: (1) the improved Fujita formula can basically solve the problem that the computed peripheral pressure and pressure gradient are too small, and also gives a reasonable distribution of pressure and pressure gradient force within the typhoon areas; (2) the improved Fujita equation, due to the impact of the environmental radius, makes the computed wind inner deflection angle smaller and wind speed greater, and in particular, changes the internal structure of the typhoon; (3) in either the wind direction or wind speed, in the range of a certain distance from the center of the typhoon, the friction force has a very small effect, but beyond this distance, both wind inner deflection angle and wind speed have a sharp increase with the gradient of friction coefficient; and at the same distance with the same friction coefficient, the improved Fujita formula gives smaller wind inner deflection angle and greater wind speed; (4) by considering the environmental influence (i.e., the introduction of a bounded environment radius parameter), the calculated typhoon periphery pressure and pressure gradient are greater, the wind inner deflection angle smaller, and the wind speed greater. When the environment radius tends to be infinite, the improved status of Fujita formula tends to be unimproved.