A solid catalyst is provided. The solid catalyst includes a core particle composed of activated carbon, lignin or iron oxide, and a plurality of hydroxyl groups and sulfonic acid groups formed on the surface of the core particle. A method for preparing a sugar is also provided. The method includes mixing organic acid and the disclosed solid catalyst to form a mixing solution, adding a cellulosic biomass to the mixing solution to proceed to a dissolution reaction, and adding water to the mixing solution to proceed to a hydrolysis reaction to obtain a hydrolysis product.