In this paper, an improved error-rate sliding window decoder is proposed for spatially coupled low-density parity-check (SC-LDPC) codes. For the conventional sliding window decoder, the message retention mechanism causes unreliable messages along the edges of belief propagation (BP) decoding in the current window to be kept for subsequent window decoding. To improve the reliability of the retained messages during the window transition, a reliable termination method is embedded, where the retained messages undergo more reliable parity checks. Additionally, decoding failure is unavoidable and even causes error propagation when the number of errors exceeds the error-correcting capability of the window. To mitigate this problem, a channel value reuse mechanism is designed, where the received channel values are utilized to reinitialize the window. Furthermore, considering the complexity and performance of decoding, a feasible sliding optimized window decoding (SOWD) scheme is introduced. Finally, simulation results confirm the superior performance of the proposed SOWD scheme in both the waterfall and error floor regions. This work has great potential in the applications of wireless optical communication and fiber optic communication.