The invention discloses a steel wire rope type marine riser tensioner fail safe function testing device and method. The steel wire rope type marine riser tensioner fail safe function testing device comprises a horizontal guide rail,a shearing simulation device and a roller seat assembly are sequentially arranged on the horizontal guide rail, the steel wire rope type marine riser tensioner is mounted on the roller seat assembly, the tensioning steel wire rope head is wound around pulleys at the front end and the rear end ofa tensioner and fixed on the shearing simulation device fixed at the rear end of the horizontal guide rail, and a shear pin in an appropriate shearing simulation device is selected and mounted in place; the steel wire rope marine riser tensioner control system is pressurized until the tensioning steel wire rope shears the shear pin of the shearing simulation device. After the shear pin is disconnected, liquid in an accumulator of asteel wire rope marine riser tensioner body rapidly pushes a piston rod of a liquid cylinder to extend under the action of expansion of high pressure gas,then whether the overall failsafe function of the steel wire rope marine riser tensioner is effective or not and whether the test is successful or not are determined according to the extending state of the piston rod of the tensioner liquid cylinder.