Overwork is an unhealthy state, causing serious harms. There are many factors causing overwork. Currently, regression analysis and correlative analysis etc. are used mostly to identify the significant variables and estimate their marginal contributions, but the relative contributions of these variables to overwork cannot be estimated, causing difficulties to identify the focuses of anti-overwork policy. One of the authors puts forward "the method of dependent variable variance decomposition" which can be used to estimate the relative contributions of the independent variables to variance of the dependent variable. Using this method we can estimate the relative contributions of the factors causing overwork. The data used are from the questionnaire survey conducted by the authors in 2017. It is concluded that according to their contributions, the factors causing overwork are: labor intensity, hours of work, job satisfaction, dinner party, wage arrears, relative pay capacity, work environment, age, grade of city, sex, type of working units.