The multi-suppliers purchasing-inventory problem are focused when the demand is tremendous and a single supplier can't meet the demand,in most cases, the demand is stochastic variable. An expectation model placing orders with multi -suppliers with capacity constraint is established, in which the demand is stochastic variable. The genetic algorithm based on stochastic simulation is the effective way to solve stochastic programming. Finally, a simple example, conclusion and potential areas for future research are presented.