The two-mode phase-field-crystal (PFC) method is used to calculate the phase diagram and to simulate the transformation of hexagonal to square structure in two dimensions. The nucleation, grain growth and dynamic feature of the phase structure transformation show that square phase prefers to be present at the juncture place of the three hexagonal grains, and swallows the hexagonal phase at grain boundary. The square grains grow and push the boundary of hexagonal grain toward the inside of hexagonal grain and then the square grains grow up and extend the area of square phase. The orientations of new square grains due to the structure transformation are nearly randomly distributed, and have no relation to those of hexagonal grains. The dynamic curve of area fraction of square phase shows the typical S shape with time increasing. The Avrami index curve shows that there are two stages in the transformation. The Avrami index n of second satge in simulation is in a range from 2.0 to 3.0, which is in good agreement with the value from the JMAK theory.