The instability occurring in OTSG (Once-Through Steam Generator) of movable nuclear power plants is presented by a multivariable frequency domain theory. As concenung coupling interactions of OTSG tubing, it is more efficient for analyzing the instability of OTSG compared the common single variable method. A mathematical model for the system is derived from the fundamental equations by using the perturbation, Laplace-transform and the nodalization techniques. The stable boundary and parameters which influence the stability of the system are evaluated through computer simulation. Numerical examples are given in the paper and the predictions of the model agree with the experimental results well.