This paper presents some valuable results of frequency doubling, considering the transversal variation of the amplitude and the depletion of the fundamental wave power. By solving three dimension-coupling wave equation, the most common expressions of second harmonic wave power are driven. The influences of the fundamental wave power and the crystal length are discussed. As a special example the expressions of second harmonic wave power at low conversion efficiency are given. As a summary, eight expressions of the second harmonic wave power at different cases are listed, in which five cases come from this paper. Finally, a new way of dealing with the intracavity frequency doubled laser is given. In our model the power loss due to SHG is considered as a viriable loss for the fundamental waye power which is circulating in the cavity. By solving the rate equations numerically, the optimum parameters of the cavity and of the frequency doubling crystal are found.