The invention relates to a method for acquiring absolute open flow of a shale gas layer based on a gas productivity index. The method comprises the following steps of: acquiring shale gas layer parameters of a tested well in a work area, acquiring shale gas layer fracturing parameters of the tested well, and acquiring open flow of the shale gas layer of the tested well by virtue of tested productivity data; sequentially calculating a gas bearing index, a fracturing transformation index and a gas production index of a tested well gas layer by virtue of the acquired parameters, and determining the model coefficient of a model AOFg=A*eB*IPG by virtue of a gas productivity index IPG calculated in a corresponding tested well by utilizing a least square method; and acquiring to-be-predicted well shale gas layer parameters in the work area, acquiring to-be-predicted well shale gas layer fracturing parameters, sequentially calculating a fracturing transformation index, a gas bearing factor and a gas production index of the to-be-predicted well gas layer according to a formula by virtue of the acquired parameters, introducing the gas productivity index into a model AOFg1=A*eB*IPG1, calculating the open flow of the to-be-predicted well shale gas layer, and outputting the calculation result. The method disclosed by the invention is adopted in 30 predicted wells of a certain shale gas field, and the average error does not exceed 20%.