Crankshaft journal roundness is an important indicator for evaluating the qualification and machining accuracy of the crankshaft. To address the problem of the inability to directly use the detection data of the crankpin for roundness error evaluation due to the revolving motion of the crankpin along the main journal during the measurement process, a roundness error detection model based on a motion coordinate system is formulated to realize the conversion and processing of the detection data of the crankpin. Meanwhile, by deeply analyzing the applicable conditions of three least squares methods used for roundness error evaluation and combining the characteristics of the sampled data, high-precision detection of theroundness error of the crankpin is achieved. Take a certain model of engine crankshaft as an example,a large sample error detection test is carried out, and the results are compared with the minimum zone evaluation，with a deviation within 1 μm.The data analysis shows the theoretical correctness andpractical feasibility of the proposed method for detecting the roundness error of the crankpin.