Abstract: Anonymous communication can hide the communication relationship for traffic flow by using some methods, so that eavesdropper can not directly know or easily induce the relation or initiator of communication Anonymity based on groups realizes the anonymity by transmitting packets forward by hosts in the group Typical prototype such as Crowds can give initiator anonymity in a certain extent by using forward probability But there is no upper bound for the length of route, under the extreme circumstance the route length will be infinite which can not be accepted by network service An anonymous communication protocol based on groups with definite route length is an improved protocol for initiator anonymity based on groups The improved protocol limits the route length The inequation indicating the relation of anonymity,length of route, proportion of compromised hosts, and group scale will be given and proved in this paper Meanwhile, a randomized function is constructed to generate random value for length of route so as to minimize the probability of initiator exposed By comparing the related data calculated under the same expected value for the length of route, it is concluded that the improved protocol has more strong anti compromised capability than Crowds