Abstract: The most important job of navigation is to determine which is dangerous target, which maybe crash our vessel in several targets.It's made humanly by navigator, usually depending on the experience of navigator, By application of fuzzy mathematics to the technique of radar marine navigation and crash avoidance, syntheticly analysing the target distance, direction, closest point of approach and time to closest point of approach, an new mathematics model for syntheticly determining dangerous target has been introduced in this paper, It can be used to describe the dangerous state of target,a indistinct notion,by quantity.