Acicular mesoporous char sulfonic acid was prepared through a one-step method of removing the template at the same time of sulfonation using ethylene tar (ET) as the carbon source and acicular nanometer magnesium hydroxide as the hard template. This method was judged as better than the two-step method of removing the template before sulfonation because it protected the mesoporous structure from damage to a certain extent. When the mass ratio of ET to Mg(OH)2 was 1:3 and carbonization temperature was 550 °C, the catalyst prepared using the one-step method had the highest activity. The obtained catalyst had an amorphous structure with a specific surface area of 446.5 m2/g, an acid density of 4.68 mmol/g, and an average pore diameter of 3.5 nm. When the catalyst was applied in the dehydration of fructose to synthesize 5-hydroxymethylfurfural (5-HMF), 97.5% fructose conversion and 80.1% 5-HMF yield can be obtained. The activity of the catalyst did not decrease after 5 cycles, which indicated that the catalyst had good stability. This research provides a promising strategy for preparation of mesoporous char sulfonic acid and comprehensive utilization of ET.