The invention provides a sulfonated cellulose water reducer based on a porous carbon sulfonic acid catalyst loaded with elemental tin, and a preparation method of the sulfonated cellulose water reducer. The preparation method comprises the following steps: mixing nano-silica microspheres with a carbon source solution, carrying out heating to remove the solvent, carrying out heating for carbonizing, and removing silicon to obtain a porous carbon carrier; then carrying out heating sulfonation treatment on the porous carbon carrier to obtain a porous carbon sulfonic acid carrier; placing the porous carbon sulfonic acid carrier in a tin dichloride solution and a ferric iron solution in sequence, maintaining the system to be acidic, and carrying out reacting, spin-drying and high-temperature calcination to obtain the porous carbon sulfonic acid catalyst loaded with elemental tin; and finally, putting dichloromethane containing activated micro-nano cellulose slurry into an intermittent reaction kettle, adding the porous carbon sulfonic acid catalyst loaded with elemental tin, carrying out pre-sulfonation treatment under rapid stirring and fast heating, then introducing a sulfonating agent, carrying out a heat-preservation reaction under high-speed stirring and slow heating, and rapidly carrying out cooling to room temperature after the reaction is stopped to obtain the sulfonated cellulose water reducer based on the porous carbon sulfonic acid catalyst loaded with elemental tin.