A training putter with a laser line projecting device can be used for putting training to improve putting skill. The device is affixed to a putter head above its sweet spot, and emits a vertical light fan which projects a line on the around normal to the face of the putter head. The line is aligned with the sweet spot of the putter head and lies on the ground to connect the top of a golf ball with a target, such as a cup or a putting hole, so that alignment can be easily visualized. The device can be detached and reattached easily without any laser alignment. Furthermore, the device can be removably mounted to a user's own putter. The heat generated in the laser diode is quickly dissipated through the metal housing, the metal putter head, and the metal shaft to the surrounding air. Since proper operating temperature of the laser diode can be maintained, the laser diode will operate properly, which means the power of the emitted light will not be decreased, and the lifetime of the diode will not be shortened.