The invention provides a forecast method and device for gas well productivity, comprising the steps of establishing dynamic changing relationship between gas well pseudopressure and shale gas output according to the linear flow production dynamic model of gas well under volume fracture, the dynamic changing relationship contains multiple basic parameters and unknown parameters; determining the major fracture semi length of the multiple unknown parameters by using the historical production data of gas well, and using the major fracture semi length to update the dynamic changing relationship; based on the dynamic changing relationship after update, obtaining parameter restraint equation via identification method of featured flow segment; and using the probability distribution model of the basic parameter and the parameter restraint equation to calculate the remaining unknown parameters and probability distribution model of productivity evaluation parameter together with the relevant credible region; using the probability distribution model and the relevant credible region to forecast the gas well productivity according to the dynamic changing relationship after update. The forecast method and device for gas well productivity can quantify the productivity forecast risk.