The heat transfer coefficient at casting-die interface is the most important factor influencing the solidification process. With a 75-ton hydraulic press machine and P20 steel die, 5-step castings of Mg-6%Al alloy with different wall-thicknesses (2, 4, 8, 12, 20 mm) were poured under various hydraulic pressures (30, 60, and 90 MPa) using indirect squeeze casting process. Thermal histories throughout the die wall and the casting surface were recorded by fine type-K thermocouples. The in-cavity local pressures measured by Kistler pressure transducers(6175A2) were explored at the casting-die interfaces of 5 steps. The casting-die interfacial heat transfer coefficients (IHTC) initially reached a maximum peak value followed by a gradually decline to the lower level. Similar characteristics of IHTC peak values were observed at 30, 60 and 90 MPa applied pressures. With applied pressure 60 MPa, the peak IHTC values from steps 1 to 5 varied from 4662 W/m² K to 8306 W/m² K. As the applied hydraulic pressure increased, the IHTC peak value of each step was increased accordingly. The wall thickness also affected IHTC peak values significantly. The peak IHTC value and heat flux increased as the step became thick. The empirical equations relating the IHTCs to the local pressures and solidification temperature at the casting surface were derived and summarized.