Compared with the most widely used H.264/AVC, the new generation of video coding standard High Efficiency Video Coding (HEVC/H.265) performs significant superiority in coding efficiency but also brings deep computational complexity and huge impact on the real-time encoding performance. To reduce the encoding time, many parallel encoding technologies are introduced. With analyzing the Slice-level parallel encoding of HEVC/H.265, we found that the unbalanced load of slices limits the speed of encoding. Therefore, in this paper an algorithm called Weighted Combination of frames Prediction (WCP) will be introduced, with referencing the information of encoded frames and combining the encoding modes and relationship of different frames in Group Of Pictures(GOP), to overcome the unbalanced allocation of Coding Tree Blocks(CTBs) for slices and improve the parallel efficiency of Slice-level encoding. The experiment results show that the proposed algorithms can reduce encoding time significantly with small increment on bitrate under the guarantee of the quality of reconstructed videos.