Abstract: The effect of cutting parameters on the machined surface roughness during high-speed milling of 20CrMnTi hardened steel with CBN straight-shank flat-bottomed end mills was studied by orthogonal test method. The effect of the cutting parameters on the surface roughness was tested by analyzing extreme differences, and the main influencing factors were clarified. The law of the influence of cutting parameters on the surface roughness was determined using the one-factor test method. A model of the relationship between cutting parameters and surface roughness based on an exponential function was created. Error analysis between the surface roughness obtained from the predictive model and the test results shows that the fixed model can more accurately predict the surface roughness. The test results showed that: the effect of each factor in descending order is the milling depth, feed per tooth, cutting speed; surface roughness increases with increasing feed per tooth and milling depth, and decreases with increasing cutting speed.