A preparation method of stichopus japonicus fermented feed capable of replacing alga is prepared by taking water caltrop, hydrilla varticillata, water hyacinth, aggregation of grass, alfalfa and common seepweed herb as the alga replacing raw material and additionally adding four, sea mud, multi-vitamin, fish meal, mussel powder, bean pulp, earthworm powder and zeolite powder with the steps of: drying all materials till the water content is less than 12%, and crushing to be 60 meshes, so as to obtain a fermentation substrate; mixing water, brown sugar, table salt and flour, adding rhizopus, trichoderma, saccharomyces cerevisiae, bacillus subtilis and lactic acid bacteria, mixing uniformly, standing and fermenting for 2h, so as to obtain a feed fermenting bacteria solution; and inoculating the fermentation substrate according to the ratio of 0.4% to 0.5% at the environment temperature of 25 DEG C to 30 DEG C, carrying out solid fermentation, and then drying and crushing, thereby obtaining the stichopus japonicus fermented feed. The preparation method has the advantages that the raw material source is stable, the cost is low, the process is simple and east to operate, and a great significance is provided in reducing culture cost of stichopus japonicas, maintaining an ecological environment, reasonably using idle resource and prompting the health sustainable development of stichopus japonicu culture industry.