For wide swath circular scanning synthetic aperture radar (CSSAR), the complex slant range history and large variant range cell migration (RCM) makes it difficult to implement imaging. To deal with the problem, the echo signal slant range equation of curve trajectory model is established by using high order approximation according to the characters of the CSSAR movements. The rangevariant range history is analyzed in details and an improved Chirp Scaling (CS) imaging algorithm for large scene CSSAR is proposed based on the derived high order approximated twodimensional spectrum. Simulation results demonstrate that the range equation under curve trajectory model is more precise and the algorithm can deal with the problem of the rangevariant RCM perfectly and realize the large scene imaging successfully.