As Minimum Variance Benchmark  considers only the time delay, which leads to the problem of the performance limitation and does not fit to PID controller’s performance assessment, PID Achievable Minimum Variance Benchmark is adopted in this paper to solve the problem of the abovementioned. The performance index  increases obviously compared with the traditional index gained through Minimum Variance Benchmark in the fourtank control system. And the process output variance is reduced by the returning parameters，too. The experimental results not only show that PID achievable minimum variance can provide a reasonable performance benchmark for the PID controller but also indicate that there is more practical significance to study the performance assessment for specific type controllers.