In this paper, we propose a two stage optimal bit allocation scheme for HEVC hierarchical coding structure. The two stage, i.e., the frame-level and the CTU-level bit allocation, are separately conducted in the traditional rate control methods. In our proposed method, the optimal allocation in the second stage is firstly considered, and then the allocation strategy in the second stage is deemed as a foreknowledge in the first stage and applied to guide the frame-level bit allocation. With the formulation, the two stage bit allocation problem can be converted to a joint optimization problem. By solving the formulated optimization problem, the two stage optimal bit allocation scheme is established, in which more appropriate number of bits can be allocated to each frame and each CTU. The experimental results show that our proposed method can bring higher coding efficiency while satisfying the constraint of bit rate precisely.