Abstract: Abstract: Taking cucumber as study object, the effects of bioorganic fertilizer on plant indices of cucumber seedlings, substrate nutrients and microbial population, and root-related characteristic parameters were investigated. Treatments include bioorganic fertilizer at four rates of dry weight (2%, 4%, 8% and 15%), and one untreated as control (no bioorganic fertilizer, CK). All parameters were finally analyzed by principal component analysis (PCA). The results showed that compared to CK on day 25 after sowing, the treatment of 8% was the optimal which significantly promoted plant growth, while the promotive effect of the treatment of 15% on plant growth became lower. The available P, K and ammonium N of the substrate increased with the increase of application dosage of bioorganic fertilizer. The urease activity increased under the treatment of 15% on day 10, 18 and 26 after sowing, and sucrase activity increased on days 10 and 25 after sowing. The bacterial and fungi population increased under treatment of 15 on day 18 and 25 after sowing. The values of fresh root weight, root dry weight, total root length, root volume, and root surface areas under the treatment of 8% were higher than those under other treatments. The highest PCA comprehensive scores under the treatment of 8% demonstrated the optimum treatment which can greatly promote cucumber seedlings growth and maintain substrate quality. The research can provide a reference for application of organic fertilizer national on cucumber seedlings.