This paper studies the problem that the theorem shows testing mean squared error's bias and variance trade-off relationship. The theorem shows that as the number of folds increases the testing mean squared error's variance becomes larger and the bias becomes smaller. This paper uses simulate a linear regression model and uses Cross-Validation to calculate the testing mean squared error. The results of computer simulation found that the larger of the number of data samples, the theorem of the bias and variance trade-off relationship is consistent with practical experience. That is, as the number of folds increases, the testing mean squared error's bias becomes smaller and the variance becomes larger. keywords: Cross-Validation, testing mean squared error