This paper studies the effects of cut-corner in the recess vaned casing treatment (RVCT) on the performance of a low-speed axial fan. The solid casing fan and fans with corner-cut RVCT have been simulated respectively. The numerical simulation shows that the introduction of the RVCT and the modification of RVCT cut-corner size bring about obvious changes in fan performance. The RVCT establishes a connection between the RVCT channel and the main flow, contributing to the extra flow circulation in the tip region and resulting in an efficiency penalty for each RVCT configuration. The cut-corner is not necessarily beneficial to the improvement of the stable operating range. With the increase of cut-corner size at the rear wall of RVCT, the improvement of stall margin firstly demonstrates a drop to −8.82% before jumping to over 60%. RVCTs with different cut-corner sizes play different roles in the flow interaction between RVCT channel and main flow, and the stall margin poses a positive relationship with the mass flow passing through RVCT. The size of the cut-corner is responsible for the flow field in RVCT. The RVCT with small cut-corner size functions as a larger blade tip clearance and results in a smaller stall margin than the solid casing fan. Therefore, the size of the cut-corner in RVCT should be large enough to dredge the flow blockage in the tip region and delay the origin of stall.