The invention discloses a hammer which is dismountable and can change the length of a moment arm. The hammer comprises a hammer head and a circular mounting casing pipe. According to the hammer which is dismountable and can change the length of the moment arm, a hammer handle II sleeves a hammer handle I, the hammer handle I extends and contracts in the hammer handle II to realize the extending or the contracting of the moment arm of the hammer, and the problem that when an object which is far away from workers needs veritical work, a conventional hammer cannot meet the using requirement is solved. A fixing base is arranged at the hammer head, a bolt is arranged on the surface of the fixing base, and the bolt can be connected with the ciruclar mounting casing pipe connected to the hammer handle I, so that the dismountable manner of the hammer head and the hammer handles can be realized; a reinforcing sleeve is arranged on the surface of the hammer handle I, and in threaded connection with the fixing base, so that the stability of the hammer head is further improved, and the problems that the hammer handle of the conventional hammer has a certain length in general and the hammer handle and the hammer head are difficult to store, carrying the hammer outside is influenced, and the conventional hammer handle cannot be extended and contracted are solved.