With the occurrence of oil spill accidents in Dalian New Port, port oil spills have been paid more attention as major marine disaster. The damages of port oil spills on port environment and marine eco-environments were obvious in recent years. Calculating oil spill risk probability is the premise of oil spill risk assessment and evaluating the spilled oil levels. This paper introduces two methods of calculating oil spill risk probability, according to analyze the information of tank farm oil spill accident, to know all the causes of accident and its logical relations, combine fault tree theory to establish fault tree model. For instance, Dalian new port, based on the probability of bottom event to calculate the probability of top event, through the analysis and calculation, the probability of tank farm oil spill accident is 0.044 every year, about 23 years happened a time.