Abstract: A basic equation governing the movement of a typhoon has been derived based on the system of primitive equations of motion in the atmosphere.The role of several forcing factors which cause the deviation of the typhoon from the steering current have been discussed based on the governing equation.What is presented in Part I is a theoretical analysis on the effects of diabatic heating and the horizontal temperature distribution. It is shown that the accelerations of a typhoon caused by diabatic heating and temperature advection coincide with the direction of the volume averaged "diabatic steering velocity" Q V and the "temperature advection steering velocity" Ar V (V is the air velocity,Q is the diabatic heating rate and Ar is the temperature advection),respectively.The precipitation(or condenation heating)on the right(left) side of a typhoon will accelerate(decelerate)the typhoon.The precipitation in the front(in the rear) of the typhoon will turn it to the left(right)of its original path. A typhoon will speed up(slow down)when it moves towards a region of cold(warm)air.