To solve the problem of the plant in the actual NC programming that the residual surface height is too large and uneven when using conventional rounded end mills, and the situation to create a second rough machining after the initial rough machining need for large cut deep processing. An efficient large cut deep milling cutter is designed which can effectively reduce the residual height through optimizing the R angle blade of conventional rounded end mill cutter. The mathematical model on the relationship of level spacing and scallop-height is established for the new rounded and the conventional rounded end mill cutter. The model parameters’ analysis shows that the new rounded end mill cutter has a good cutting effect. The cutting simulation of new and conventional rounded end mill suggests that the former processing efficiency can be improved 2.4 times. The designed tool is grinded. Compared to conventional rounded end mill cutter, the actual processing experiment shows that the surface residual height can be reduced 1.96 times with the new rounded end mill cutter.