Based on the diversity and randomness of arriving vessels, a navigation operation system simulation model of coastal bulk cargo port area is establishe d to analyze the impact of vessel combination pattern on waterway capacity. The simulation result shows that vessel combination pattern has significant effect on waterway capacity. The weighted mean of arriving vessels DWT is defined as characteristic DWT in this paper. There is an exponential relationship between waterway capacity and characteristic DWT; and the longer the navigation duration, the higher the sensitivity of waterway capacity to vessel combination pattern.