The critical points of Ac1, Ac3, Ar3 and Ar1 of Nb-microalloyed steel Q345B, Q345C and pipeline steel X70 were studied by PCY thermal expansion instrument (thermal expansion coefficient apparatus) with 2 ℃ / min heating/cooling rate, and the thermal expansion coefficients were calculated according to the expansion curves of the experimental steel. The results showed that the difference values of critical points of Ac1 and Ac3 of Q345B and Q345C steel were not large in heating process, which were 8 ℃ and 3 ℃, but in cooling process the temperature differences of Ar1 and Ar3 were 20 ℃ and 25 ℃ respectively. As there was more microalloyed elements of Nb, V, Ti, etc. containing in X70, the dissolution and precipitation of carbon nitride brought about greater impact on the phase transformation of steel, the critical point of X70 is much higher than that of Q345B and Q345C. There is no much difference in the expansion coefficients of the three tested steel .