A method for determining a weight-adjusting parameter in a variable-weight vulnerability assessment method for water-outburst from coal seam floor, comprising the steps of determining a dominant factors and a constant-weight weighting value, further comprising the following steps: 1) building a state variable-weight vector formula; 2) selecting or giving an assessment unit in accordance with constraint conditions; 3) determining an optimum variable-weight weighting value of the selected assessment unit; and 4) solving a value of the weight-adjusting parameter according to a parameter solving model. The method for determining a weight-adjusting parameter in assessment and prediction of vulnerability for water-outburst from coal seam floor by means of a variable-weight model is proposed at the first time. In this method, an optimum variable-weight weighting value of the selected assessment unit is set, and then a value of the weight-adjusting parameter is solved according to a built parameter solving model. With practical application testing, the weight-adjusting parameter determined by this method can effectively reflect the controlling effects on water-outburst of the multiple dominant factor index values in various combined states, and can effectively improve the precision of assessment and prediction of vulnerability for water-outburst from coal seam floor.