With the increase in strength of pipeline steels manufactured by thermomechanical control process (TMCP), the softening of the welding heat-affected zone (HAZ) becomes another important factor affecting the properties of welded steel pipes and the safety of pipeline operation. In this work, based on the actual welding process of steel pipes, the strength, phase transformation, and microstructure of the HAZ of six pipeline steels with different chemical compositions were studied by using a thermomechanical simulator, and the effect of chemical composition on the softening of HAZ was discussed. Results show that the strength of HAZs is significantly influenced by the peak temperature, and the softening zone mainly occurs in fine-grained HAZ (FGHAZ) when peak temperature is 900~1000 °C. Meanwhile, the degree of softening is also affected by the chemical composition of the steels. The effects of peak temperature and chemical composition of the steels on the strength of the HAZs when the peak temperature is over Ac₃ are attributed to their effect on the austenite transformation during the heating process, and then the effect on phase transformation during the cooling process and final microstructure. The strength of the HAZs is linearly related to the beginning phase temperature during the cooling process, and the strength of sub-HAZs at the same peak temperature is linearly related to the value of carbon equivalent (Ceq) of steels. Therefore, controlling the appropriate value of Ceq is necessary to improve the softening of HAZs for high-strength pipeline steels.