The invention discloses a biogas fermentation tank top scum shell breaking device and a shell breaking method. The shell breaking device comprises a guide cylinder and a stirrer. The guide cylinder is arranged in a biogas fermentation tank. The upper and lower ends of the guide cylinder are in a shape of a horn mouth. The middle of the guide cylinder is cylindrical. The top surface of the horn mouth at the upper end of the guide cylinder is lower than the liquid surface in the biogas fermentation tank. The stirrer comprises a motor, a reducer, a stirring shaft and stirring blades. The motor is arranged on the top of the biogas fermentation tank. The stirring blades are located below the horn mouth at the lower end of the guide cylinder. One end of the stirring shaft is connected to the motor through the reducer and the other end passes through the inner cavity of the guide cylinder and is connected to the stirring blades. The device can effectively solve the problem of caking of scum top in a biogas fermentation process, improves a raw material use rate in biogas fermentation and solves the problem of discharge of easily caked raw materials in biogas fermentation.