The invention relates to a landscaping garbage decomposing agent and a preparation method thereof. The landscaping garbage decomposing agent comprises, by weight percentage, 10-30% of lignin degrading enzyme, 10-30% of cellulose degrading enzyme, 20-40% of slurry and the balance of water. The preparation method includes: using a culture medium to culture microorganism strains, placing various strains into a constant-temperature shaker for oscillation culture for 24-48 hours, and adding sterile water for diluting after the culture. The landscaping garbage decomposing agent is low in cost, fast in degrading and capable of efficiently degrading landscaping garbage to allow the same to be decomposed into organic fertilizer and has profound significance and actual value.