In order to improve the bit-allocation effect of the Largest Coding Unit (LCU) and the parameter-update precision ( α、 β), in the rate control algorithm of H.265/HEVC based R-λ model, an optimized rate control algorithm was proposed. By utilizing the existing encoding basic unit, bit allocation was carried out, and the parameters ( α, β) were updated by using the coding distortion degree. The experimental result shows that in the constant bit rate case, compared to the HM13.0 rate control algorithm, three component PSNR gain improves at least 0.76 dB, the coding transmission bit reduces at least by 0.46%, and the coding time reduces at least by 0.54%.