The screen content coding (SCC) extension of High Efficiency Video Coding adopts three modes, the conventional intra mode of HEVC, the new intra block copy mode and palette mode, to improve the coding performance of screen content videos. However, the exhaustive search for the optimal mode among the three mode candidates brings significant computational burden to a SCC encoder. This paper proposes an efficient mode decision algorithm for SCC by content analysis. Since screen content videos contain both stationary coding units (CUs) and dynamic CUs, two different techniques are proposed. For dynamic regions, the high gradient pixels and the background color are jointly analyzed to perform CU type classification, and then different mode candidates are checked adaptively according to the CU type. For stationary CUs, the information from the collocated CUs is utilized to predict the optimal mode. Experimental results show that the proposed algorithm achieves 40.05% computational complexity reduction on average with 1.40% Bjøntegaard delta bitrate loss under All Intra configuration.