The invention discloses an automatic punching machine for a radiator and a working method of the automatic punching machine. The automatic punching machine comprises a punching machine body, a punching mechanical arm, a cooling fin cutting device and a cooling fin insertion device. Cooling fins are cut through the cooling fin cutting device. The punching mechanical arm can be used for gripping thecooling fins. The cooling fins are put into the cooling fin insertion device through the punching mechanical arm. The punching machine body comprises a lower base, a punching rack, a lower die holder, an upper die holder, a punching machine head, guiding pillars, a first driving motor, a first electric cylinder and a tightly-pressing assembly. The punching rack is arranged on the lower base. Thefirst driving motor is arranged on the upper end face of the punching rack. The first electric cylinder is fixed to the lower end face of the punching rack and connected with the first driving motor and the upper die holder. The punching machine head is arranged on the lower end face of the upper die holder. The lower die holder is arranged on the upper end face of the lower base. The guiding pillars penetrate through holes in four corners of the upper die holder.