The present disclosure provides a new mutant of Bacillus thuringiensis and a method for preparing microalgae growth promoter. ITRI’s microalgae technology has found a B. thuringiensis mutant strain capable of stimulating the growth of microalgae, where metabolite from the mutant allows the productivity of Nannochloropsis sp. to increase by 163%. In order to commercialize this technology, compound X capable of stimulating the growth of microalgae has been developed based on bio-mimic application, thereby enhancing the productivity of Nannochloropsis sp. to reach 121%. Moreover, tests showed that this microalgae growth promoter also has the effect of stimulating growth for Haematococcus pluvialis, Chlorella sp., Tetraselmis sp., Isochrysis galbana and Dunaliella sp., and can be vastly applied to different fresh water as well as sea water microalgae. In addition, ITRI’s technology multiplies the productivity of microalgae under conditions without altering existing equipment, nutrient source and cultivation criteria, and can be used in conjunction with other technologies, thereby reducing the production cost for microalgae.