Abstract: The security problems of spatiotemporally chaotic cryptosystem are studied.This kind of systems has the weakness that the high-dimensional receiver degenerates to be one-dimensional under constant driving,therefore,is no more sensitive to the slight perturbation on the driving after convergence.Based on this weakness,a slight perturbatioin attack method is proposed: choosing a constant driving,thus the receiver systm degenerates to uniform object state;adding a slight perturbation to the constant driving,and making the distance between pre-perturbation state and post-perturbation state in a small extent.This attack disabled the effects of INT and MOD operations and exposed the state variables.The experimental results show that this method can successfully get the key to some precision with relatively small computation cost.