Abstract: AP1000 nuclear power plants use Mechanical Shim（MSHIM） operation. At present, few studies have been done on impacts to ex-core operation parameters from MSHIM operation. Control rod insertion and cold leg temperature reduction processes were simulated, and flow streaming and ex-core detector shadowing effects due to MSHIM operation for AP1000 plants were analyzed in this paper. The results show that: M banks can obviously influence the flow streaming and ex-core detector shadowing effects, while AO bank has negligible influence to these effects; cold leg temperature reduction results in obvious ex-core detector shadowing effect which is insensitive to the control rod position. This paper proposed that following control rod positions should be included in related startup tests: 130, 14,-66,-116,-296 and-436 steps. During full power operation, tests at-436 steps could be executed at 130 steps to avoid a low power margin.