Based on the ray-tracing theory,a calculation model for X-ray transmission in a tapered X-ray capillary was set up. Using the calculation model,systematic theoretical studies on the transmission characteristics of a tapered capillary were performed. Calculating the intensity distribution,the conclusion was got that the size of X-ray source and the entrance diameter of the tapered capillary affected the form of intensity distribution. We got the change rules of power density gain K with R0 and f2. Taking the equivalent distance Leq as a standard,the optimal design of tapered capillary was discussed.