Aimed to ensure the correct transmission of most important data in case of the channel's performance deterioration, this paper presents a novel unequal error protection scheme based on LT codes. Firstly, the UEP performance deterioration of the alternative UEP scheme, which is caused by the simple rounding operation, is analyzed, and an improved scheme that can avoid the UEP performance deterioration is proposed. In this scheme the bidirectional rounding operations according to different probabilities in degree computations of most important bits are introduced, and the fixed degree skew coefficient is revised to a function of priority and degree of output nodes. Then the upper and lower bounds on the ML decoding error probabilities of MIB and LIB over the BEC are deduced. Finally the UEP performance on the BP decoding over erasure channel is simulated. The theoretical deduction and simulation results show that the proposed UEP scheme has better UEP performance and more flexibility in code design than the alternative UEP schemes.