According to the longitudinal matching requirement of the HIRFL two cyclotrons, a scheme of the new B1 buncher has been introduced. Both the moving short-circuit plate and the two movable capacity plates are employed to meet the large frequency range (22~54MHz) requirement. With the help of three-dimensional electromagnetic field computation code MAFIA, the frequency range, Q factor, power losses and surface current distribution of the B1 cavity have been computed. The errors in computation and the feasibility of the scheme have been discussed also in this paper.