Aimed at coarse alignment of Strapdown Inertial Navigation System (SINS), the theory of reference vectors based on gravity vector has a problem of collinearity because the alignment time is transitory and the angle of gravity vectors turning in inertial space is little. The condition of ill-conditioned matrix theory was used to study the problem, the effect of alignment time on collinearity was confirmed. The simulation results show that the problem can be ignored after 1 minute, and the coarse lignment can be finished.