Abstract: The Compton detector, with fast time response, linear range over many orders of magnitude and low sensitivity to X-ray fluxes, can be used to measure the dose of time resolved X-ray, whose sensitivity is decided by the materials and dimensions of its collector, dielectric and metal case. Since the detector dimension is energy dependent, there will be an optimal dimension for a detector to measure bremsstrahlung X-ray dose. Monte-Carlo method was used to design a Compton detector which will be used to measure the bremsstrahlung X-ray dose produced by the Dragon-Ⅰ accelerator. The bremsstrahlung spectrum of Dragon-Ⅰ was computed by MCNP program, as well as the Compton electron number as a function of plexiglass thickness, aluminum case thickness and plumbum collector thickness. According t