To solve the problem that the low capacity in hot-spots and coverage holes of conventional cellular networks, the base stations (BSs) having lower transmit power are deployed to form heterogeneous cellular networks (HetNets). However, because of these introduced disparate power BSs, the user distributions among them looked fairly unbalanced if an appropriate user association scheme hasn't been provided. For effectively tackling this problem, we jointly consider the load of each BS and user's achievable rate instead of only utilizing the latter when designing an association algorithm, and formulate it as a network-wide weighted utility maximization problem. Note that, the load mentioned above relates to the amount of required subbands decided by actual rate requirements, i.e., QoS, but the number of associated users, thus it can reflect user's actual load level. As for the proposed problem, we give a maximum probability (max-probability) algorithm by relaxing variables as well as a low-complexity distributed algorithm with a near-optimal solution that provides a theoretical performance guarantee. Experimental results show that, compared with the association strategy advocated by Ye, our strategy has a speeder convergence rate, a lower call blocking probability and a higher load balancing level.