Fork-join is a basic query processing model in shared-nothing parallel database systems. A query Q is decomposed into a number of sub-queries, and each of which is processed independently on a processing element(PE), then all the results of sub-queries are “joined” and returned as Q’s results. In this scheme, the query processing time of Q depends on the time of its last finished sub-query. Though it is very important, there are few works studied the performance of multi-query processing on Fork-join model. In this paper, we present a model, called CSD(Coefficient of Scheduling Distance), to evaluate the influence of independent PE scheduling on the total completion time . All the experiments are conducted on DBroker, a large-scale production system for network security management. The experimental results show that the CSD model is accurate and effective.