The invention relates to a liquid tank device, a 3D printer and a mounting method. The liquid tank device comprises a base, a material cylinder, a light-transmitting plate and a sealing ring, whereinthe material cylinder is arranged on the base, a first light-transmitting hole is formed in the base, a second light-transmitting hole is formed in the material cylinder, the light-transmitting plateis clamped between the base and the material cylinder, a projection of the first light-transmitting hole and a projection of the second light-transmitting hole at a first surface of the light-transmitting plate is at least partially overlapped, the material cylinder comprises an extending part, the first surface of the light-transmitting plate is abutted against the extending part, the extending part is provided with a groove used for containing a sealing ring, and the sealing ring protrudes out of the groove, and presses tightly the light-transmitting plate on the base. According to the liquid tank device, the parallelism of the light-transmitting plate and the base is effectively guaranteed, and the light-transmitting plate can be conveniently replaced in the later stage.