A globally asymptotically stable nonlinear control law for longitudinal motion of underwater vehicle was proposed using the modified adaptive inversion technique to ensure the global asymptotical stability with the presence of the parameter uncertainty. To solve the problem of hard bounded climbing or diving velocity, a bounded stabilizing function in atan form was chosen, and the Lyapunov function was properly designed to reduce the effect of the large depth track error on the rudder input. The global asymptotical stability and the adaptability to the unknown parameters were proved by theory analysis and simulation study.