Abstract: According to the big filtration lost problem occurred in the hydraulic fracturing on the seam reservoir before the coal bed methane in production, from the coal reservoir features and the fracturing operation, the paper analyzed the factors affected to the comprehensive filtration lost coefficient.The results showed that the f actors affected to the comprehensive filtration coefficient of the seam reservoir mainly would be dynamic permeability rate, coefficient of pore fluid in coal seam, porosit y and others.In the fracturing operation, factors affected to the comprehensive filtration lost coefficient would be the fracturing liquid viscosity and others.With the relate d formula, in combination with the typical calculation method of the comprehensive filtration lost coefficient, each factor cofficient, filtration lost affected to the compreh ensive could be quantitatively evaluated.The analysis calculation showed that the evaluation method would have reliability and practices.