Video encoding is based on the DPCM framework where temporal prediction coding introduces Rate-Distortion (RD) dependence. The RD operating point of the current unit depends on the particular choices of RD points of its reference units. Unfortunately, common Lagrangian optimization method based Rate-Distortion Optimization (RDO) for video coding is based on an independence assumption which omits the RD dependences, and thus compromises the RD performance. In this paper, we revisit the Lagrangian optimization method based RDO for dependent video coding. A theoretical RD dependence decoupling method based on independent distortion decomposition is firstly presented. After the discussion of reasonability of the theoretical decoupling method, the practical One Step Ahead Decoupling Strategy (OSADS) is proposed. After implemented on the HEVC encoder, the strategy achieves average 2.1% BD-rate saving compared with the HM encoder under the same low-delay P configuration.