Optimal multiprocessor real-time scheduling can achieve full system utilization with implicit-deadline periodic task sets. However, worst case execution time (WCET) analysis is difficult on state-of-the-art hardware/software platforms due to the complex hierarchy of shared caches and multiprogramming. The actual case execution time of each task is usually shorter than its WCET and imprecise computation is an effective method to make better use of the remaining processor time. Semi-fixed-priority scheduling is real-time scheduling that supports imprecise computation and multiprocessors but conventional semi-fixed-priority scheduling algorithms are not optimal. This paper proposes an optimal multiprocessor semi-fixed-priority scheduling algorithm that supports imprecise computation. The proposed algorithm, which integrates Reduction to Uniprocessor (RUN) for Rate Monotonic with Wind-up Part (RMWP), called RUN-RMWP, is superior to Partitioned RMWP algorithm in terms of schedulability analysis. Simulation studies show that RUN-RMWP has a few more preemptions/migrations compared to RUN but confirms its optimality even though conventional semi-fixed priority scheduling algorithms are not optimal.