This paper aims at the problem of fast stability control of amphibious robot after state switch. A novel method named as generalized multivariable adaptive super-twisting algorithm (GMASTA) is proposed for the finite time attitude control of amphibious robot. In the proposed method, the standard multivariate super-twisting algorithm is improved by introducing a linear term and a higher power term, in order to reduce convergence time and chattering. In addition, the adaptive gain is employed based on a disturbance observer which can avoid a conservative overestimation of the disturbance. For the designed attitude controller, the finite time stability is proved by Lyapunov method. Two numerical examples were carried out. The results from the first example show that GMSTA is better than MSTA in convergence and robustness. The other example is used to illustrate that the disturbance observer based on the GMASTA can realize the accurate observation of the integrated interference. Compared with PID, the proposed method based on GMASTA can converge faster and has better robustness.