In this paper, we purpose a novel approach to analyze exact cell loss probability of Shared Buffer ATM switch. In a N X N ATM switch whose input ports and output ports share one buffer, the number of arriving cells from input ports at one time slot is less than or equal to N. If these cells were destined to n output ports, no cells will be destined to the remaining N-n output ports. In the extremism, all arriving cells were destined to one output port, no cell will be destined to the other N-1 output ports. So, the arriving cells at each time slot from the input ports are destined to the output ports correlatively, but not independently. For this reason, the analyzing result on the assumption that all output cell queues are independent one another is larger than actual result. We complete our analysis by three steps. First, considering the case that there are cells arriving from input ports but not cells remove from output ports, we can get one-step state transition probability matrix Pa; Second, considering the case that there are cells removing from output ports but not cells arrive from input ports, we can get one-step state transition probability matrix Ps; Third, considering arrival process and service process are independent, the accurate one-step state transition probability matrix P of cell's number in the buffer is derived, and then cell queue length steady probability distribution can be found. We can compute the accurate buffer capacity below certain cell loss probability. Computer simulation proves that the approach is efficient and exact.