The formula of transmitted power of axis-symmetrical modes is obtained from the integrate solution of wave equations. It contains the influence of the incident light and can be used to compare the powers of different modes. Simulating a practical light spot on the end-face of an optical fiber with a Gaussian-spot, the influences of focus situation on power distribution among different modes are discussed. It is shown that by adjusting frequency and focusing situation properly, some modes can be enhanced and others can be restrained.