Abstract: A weight based soft constraint tuning method is introduced to deal with the infeasibility problem of steady-state target calculation. In order to show the priorities of the constraints in soft constraint tuning, a strategy for descending priority is proposed. The constraints with high priority will be satisfied firstly. Furthermore, a comparison of descending priority strategy with the weight based strategy and the ascending priority strategy is made from the feasible zone's size and computation complexity analysis. The theoretical analysis shows that the advantage of descending priority strategy relative to descending priority strategy and the weight based strategy. Finally, a simulation of Shell's standard heavy oil fractionator is done to verify the validity and advantage of the descending priority strategy.