The invention discloses a movable steel base for constructing a prefabricated beam and a standard joint thereof. The standard joint comprises a beam bottom formwork and a supporting seat connected with the beam bottom formwork by means of bolts; the lower part of a first steel plate in the beam bottom formwork is symmetrically provided with first channel steels positioned on two sides of the first steel plate, and the middle of the first channel steels on the two sides is fixedly connected with a first angle steel; second angle steels in the supporting seat are symmetrically arranged at the bottom and contacted with the ground, third angle steels are arranged above the second angle steels, second channel steels are arranged above the third angle steels, four angle steels are arranged between the second channel steel on one side of the supporting seat and the second angle steel on the other side, and third steel plates are arranged at the front and rear ends of the supporting seat; andsecond steel plates are arranged between the first channel steels and the second channel steels. The movable steel base is assembled by using the standard joint by means of the bolts. The standard joint has the advantages of short construction time and low cost, and can be reused at different construction sites.