This work presents a conservative phase-field lattice Boltzmann method (LBM) for boiling heat transfer at high density ratios. This method employs the LBM for modeling unsteady flows, the finite difference method for solving the heat transfer equation, and the conservative phase-field method as the interface tracking model. In order to model the boiling process, the conservative Allen–Cahn model is used. The proposed method has been successfully validated by using a bubble rising in a viscous fluid and the film boiling. In order to demonstrate its capability in modeling complex flows, the method is applied to study the flow boiling heat transfer in a microchannel with multiple nucleation cavities. Simulations are performed by varying the Reynolds number (Re) from 50 to 150 and the Stefan number (Ste) from 0.023 to 0.16. Furthermore, the contact angle (θ) is examined as a design parameter with values of 60° and 90°. The results show that the phase-field LBM successfully achieves the highly unsteady flow boiling heat transfer in microchannels at a high density ratio. It accurately estimates the differences in thermal performance between bottom and upper walls for various excess temperatures and Reynolds numbers, where the boiling heat transfer rates on the bottom wall are higher than those achieved on the upper wall. That is due to the high vapor accumulation rate on the upper wall. Furthermore, the results show the accuracy of thermal fluid–solid interactions at different surface contact angles, and the findings recommend hydrophilic surfaces for higher thermal performance due to the decrease in vapor bubble attachment and deposition over the heated surfaces.