The invention designs a key stream generator which can be used for computing the output pseudo-random sequence with random length under the condition of low resource consumption, and the generating algorithm of the key stream generator can be approved to be safe; the safety of the key stream generator is based on the difficulty of solving multivariate quadratic equation set within a finite field and can meet the design demand of the approved safe key stream generator, so that the key stream generator can be applied to the field with higher requirement for the safety. In one instance, the key stream generator is defined by multivariate multinomial set on one GF (q), and the q is large prime number. By selecting proper parameter, key stream with random bit can be generated at higher speed according to the length of plaintext, and the safety level of 2<80> can be reached, so that the key stream generator can meet the application need of most current passwords.