In order to study the influence of the cutting parameters to the surface roughness of 45 quenched and tempered steel in Numerical Control (NC) turning, the turning test is designed based on the single factor experimental method with carbide tool. The machining surface roughness values were obtained by the surface profiler and analyzed. The result indicates that: within the range of cutting parameters used in the experiments, the surface roughness value is increased almost linearly with the cutting feed rate increased. The surface roughness value is decreased when the cutting depth increased from 0.05 mm to 0.10 mm, and increased when the cutting depth from 0.10 mm to 0.20 mm. The surface roughness value is decreasing when the cutting speed from 500 r/min to 1 000 r/min, and increasing when the cutting speed from 1 000 r/min to 1 400 r/min. The research has a certain guiding significance on the actual processing and can be as a theoretical support for rational selection of turning 45 quenched and tempered steel.