For a fast ant system (FANT),the strategy for jumping out of local optimal solution was analysed,and the reason that the algorithm is easy to occur stagnation phenomenon was pointed out.Through the improvement of the solution construction step in the algorithm and the introduction of a variable parameter,a new algorithm for quadratic assignment problem-preprocessing fast ant system (PFANT) was proposed.The new algorithm mends FANT's some shortcomings such as stagnation,extends the search area of FANT iterative optimal solution neighbourhood and improves the solution quality.