Assuming the unit weight of soil has no influence on the slip-line field, the formula of bearing capacity of strip footing is got by slip-line method, in which the three bearing capacity factors Nq, Nc and Nγ don’t effect each other and Nq, Nc are the same as the published results. The upper-bound solution to bearing capacity of strip footing is deduced in the corresponding velocity field of strip footing to the slip-line field, which is the same as what is got using slip-line method. The slip-line result is accordingly proved to be the upper-bound solution. The bearing capacity factor Nγ is compared to those by other methods. The reasons that lead to the error by present method are analyzed and a modified formula is proposed.