This study aimed to optimize the preparation conditions of Trichoderma harzianum inoculant and evaluate its degradation efficiency of alachlor in aqueous environments, as well as the toxicity of the resulting degradation products. Single-factor experiments determined the optimal preparation parameters as follows: temperature 28.5 ℃, pH 7.7, 5% (w/w) sodium alginate as a drying protectant, 1% ascorbic acid as a UV protectant, 5.2 g rice bran as carrier, and a cultivation period of 6.5d. Degradation efficiency was measured by high-performance liquid chromatography. Under aqueous conditions, the highest degradation rate reached 97.35% when the degradation period was 5 d, temperature 30.2 ℃, pH 7.1, and initial alachlor concentration 50.0 mg L-1. Seed-germination assays showed that seeds exposed to degradation products from the inoculant exhibited significantly higher germination rates and shoot lengths than those treated with free-cell degradation products; both treatments outperformed the alachlor-only control, confirming that the inoculant effectively reduces the environmental toxicity of alachlor.