The mathematical model of optical packet switching architecture based on feedback FDL contention resolution is stablished and analyzed. The priority of the packets from FDLs is higher than those from input fibers in this model. The formulation of packet loss probability is deduced through neglecting the degradation of optical signal and the crosstalk in optical switch matrix. The packet loss probability is lower in low load than in high load. This is due to the nonlinear distribution of the probability of arriving packet from feedback FDL vs the load. With the increment of the load,the probability of arriving packet increases very slowly in low load and very fastly in high load. There exists no limit in packet loss probability by adding the number of FDL to improve the performance. The characteristic is different from the contention resoluton based on wavelength conversion that exits a limit. It will be very useful to comine wavelength conversion and FDL to break through the limit.