The second generation Audio Video Standard (AVS2) adopts the flexible partitioning structure, which recursively divides the coding tree unit (CTU). The prediction models (PM) in inter prediction include PSKIP, P2Nx2N, P2NxN, PNx2N, PHOR_UP, PHOR_DOWN, PVER_LEFT and PVER_RIGHT. Each PM needs to perform integer motion estimation (IME) and fractional motion estimation (FME) in the motion estimation (ME) process. These methods adopted by AVS2 contribute significant coding efficiency while generating great encoding complexity. In order to reduce the encoding complexity, an adaptive inter mode decision fast algorithm is proposed in this paper from two aspects. Firstly, we statistically analyze the PMs in both spatially and temporally adjacent positions and establish a mode complexity (MC) measure metric. Through experiments we know that the complexity of inter mode decision decreases with the reduction of MC. Therefore, MC can be utilized to guide skipping some PMs with low occurrence probability. Secondly, a fast algorithm based on the parent prediction unit (PU) is proposed. FME in child PUs is skipped when the parent PU’s MV from IME is equaled to FME. Experimental results show that, compared to the AVS2 reference software RD17.0, the proposed fast algorithm reduces total encoding time by an average of 21.2% while the performance loss is negligible.