The effect of carbonitrides on the growth of austenite grains in high-strength low-alloy (HSLA) steel was investigated through microstructure analysis and high-resolution dilatometry. The austenite grain growth at various temperatures was measured in situ using high temperature laser scanning confocal microscopy. Two kinds of precipitates with different morphologies containing large amounts of Ti and Nb were observed in the austenitized samples at different temperatures. The smaller Nb-rich spherical carbonitrides dissolved at the higher temperature, while the lager Ti-rich rectangular carbonitrides have a higher thermal stability and can pin at the austenite grain boundary. This study reveals that the dissolution of Nb in austenite causes the rapid growth of austenite grains and slows down the ferrite phase transition, which is beneficial to the formation of bainite. A kinetic model of austenite growth considering the dynamic dissolution of different types of carbonitrides is proposed. The results of the model show the same overall trend as the experimental results, and the individual differences can be explained. The proposed model can be used to predict the austenite grain size during the austenitizing process of HSLA steel and can reflect the austenite grain change throughout the process more continuously.