Here, we reported an excellent carbon-based solid acid catalyst for the catalytic synthesis of biodiesel, which was prepared by carbonizing a mesoporous phenolic resin and then followed by sulfonation with concentrated sulfuric acid. The influence of carbonization temperature on the pore structure and acidity was studied. Fourier transform infrared spectroscopy, energy dispersive spectrum analysis and an indirect titration method were used to demonstrate the successful modification of –SO₃H groups on the carbon surface and determine the acidities of catalysts. N₂ adsorption–desorption and transmission electron microscopy were used to characterize the mesoporous structure and pore structure parameters. These results indicated that the sulfonated catalyst carbonized at a low temperature (400 °C) showed the highest acidity of 2.21 mmol H⁺ g⁻¹, meanwhile, retaining a mesoporous structure and relatively large surface area. The esterification reaction of oleic acid with methanol was employed to evaluate the performance of catalysts. The sulfonated mesoporous carbon catalyst exhibited a highly efficient activity, above 95% conversion of oleic acid with a 30 : 1 methanol/oleic acid at 70 °C for 3 h. Experimental parameters, including the molar ratio of reactants, reaction time and reaction temperature, were optimized and a superior recycling property was presented after five consecutive cycles.