The invention provides a synthesis method of a polysiloxane modified carbon quantum dot. The method comprises the following steps: firstly, feeding citric acid into deionized water, stirring the citric acid to enable the citric acid to be sufficiently dissolved; secondly, feeding polysiloxane emulsion into the citric acid solution, uniformly stirring the polysiloxane emulsion, and then laying thepolysiloxane emulsion into a 120 to 180 DEG C constant temperature drying box, so as to be reacted for 4 to 12h, to obtain light yellow solution; thirdly, performing suction filtration on the light yellow solution, so as to obtain yellow light transparent filter liquor, and diluting the yellow light transparent filter liquor for 100 times through deionized water, so as to obtain the polysiloxane modified carbon quantum dot. The quantum dot has the advantages of cheap synthesis materials, high simpleness in operation, high fluorescence intensity, good stability, good biocompatibility, no pollution and no toxic action, and has great potential application value in terms of fluorescence probes, biological markers and imaging and pathological detection. The fluorescence quantum yield of the quantum dot can reach 80 percent or more.