Bolts are widely applied to lap joints in many industrial fields. Due to harsh environment and low quality of operation, bolt looseness may appear during service. For lack of an understanding of the Lamb wave propagation process across bolted joints, most of the existing methods exploit some basic characteristics such as the transmitted energy as the looseness index, which is insensitive to bolt looseness at the early stage. To take full advantage of the information in the Lamb wave propagation and improve the detection performance, this study proposes a method for bolt looseness detection based on the phase change of Lamb waves during the propagation process. Firstly, the contact status is investigated based on the distribution of contact press, and the change of contact area along with bolt torque is analyzed. Subsequently, the propagation process of Lamb waves across bolted joints is revealed, based on which the monotonic relationship between the phase delay of Lamb waves and the extent of bolt looseness is derived. Thus, a looseness index based on phase change can be established to detect bolt looseness. Finally, the experiment is carried out in a bolted joint composed of two aluminum plates. The phase change in received signals conforms to the analyzed propagation mechanism of Lamb waves in bolted joints and verifies the effectiveness of the established bolt looseness index. Compared to the traditional methods based on the transmitted energy of Lamb waves, the proposed method is more sensitive to the change of bolt torque, especially at the early stage of bolt looseness.