The invention discloses a mepiquat chloride effervescent tablet and a preparation method thereof. The mepiquat chloride effervescent tablet is made of the raw materials including the substances based on the parts by mass: 0.1-60 parts of mepiquat chloride, more than 0 part and less than or equal to 40 parts of water-blocking agent, 3-30 parts of acidic effervescence agent, 3-30 parts of alkaline effervescence agent, 1-10 parts of binding agent, 1-20 parts of dispersing agent and 1-50 parts of glidant. The mepiquat chloride effervescent tablet can be prepared by a direct drying tabletting method or a wet granulation and secondary tabletting method, wherein the direct drying tabletting method can be used for preparing the mepiquat chloride effervescent tablet by mechanically grinding auxiliary agent, evenly mixing the auxiliary agent and the raw mepiquat chloride and directly tabletting; and the wet granulation and secondary tabletting method can be used for preparing the mepiquat chloride effervescent tablet by drying the raw mepiquat chloride, grinding the auxiliary agent, preparing acidic and alkaline soft materials by mixing, bonding and pelleting, and finally tabletting. The water-blocking agent is introduced to solve the problem of moisture absorption of active ingredient of mepiquat chloride solid preparation; furthermore, the prepared mepiquat chloride effervescent tablet is a new formulation product having the characteristics of rapid disintegration, completely dissolving out of the active ingredient, good suspension property of insoluble auxiliary agent and accurate and quantitative field operation.