Abstract: In order to clarify the reasonable application amount of organic fertilizer on greenhouse watermelon and build ascientific fertilization system for greenhousesoil, a field experiment was carried out to study the effect of application amount of commercial organic fertilizer on soil nutritionand nutritional quality of watermelon. Five treatments were set up, including commercial organic fertilizer of 12.5 t/hm2(CK), 29.2 t/hm2 (M1), 41.7 t/hm2 (M2) and 54.2 t/hm2 (M3), oil residue of 16 t/hm2(OD) as base fertilizer. The results showed that Vc and soluble sugar content of watermelon increased first and then decreased with the increase of organic fertilizer application amount. Vc content reached the highest in M1 treatment and soluble sugar content reached the highest in M2 treatment.Compared with CK, mineral nitrogen content in 0-20 cm soil layer in expanding period and after harvest and in 20- 40 cm soil layer during the whole sampling period of OD treatment was significantly higher. The soil mineral nitrogen content in 0-20 cm soil layer of M2 and M1 treatment increased by 32%-60%in comparing with that of CK treatment before transplanting. The available phosphorus content of M1 and OD treatment in 0-20 cm soil layer was in the suitable range of 60-90 mg/kg. Compared with CK, the application of commercial organic fertilizer and oil residue could significantly increase soil available potassium by 1.0 to 3.7 times in 0-20 cm soil layer before transplanting. Compared with that of other treatments, soil electrical conductivity of OD treatment reached the highest in 0- 40 cm soil layer. Therefore, commercial organic fertilizer application amount of 29.2-41.7 t/hm2could promote the quality of watermelon and reduce environment pollution.