Larix gmelinii is among the most important timber species in northeastern China due to its cold hardiness, drought resistance and rapid growth. It is ecologically and economically very important, but is seriously affected by Lymantria dispar L., a defoliating insect pest. Plants, including trees, employ an array of physical and chemical constitutive defense mechanisms which play an important role in protection from insect herbivory. Chemical defenses include secondary metabolites and proteins. The primary defense proteins consist of protective enzymes, e.g. peroxidase (POD), superoxide dismutase (SOD) and catalase (CAT); defense enzymes, e.g. polyphenol oxidase (PPO) and phenylalanine ammonia-lyase (PAL); protease inhibitors, e.g. trypsin inhibitor (TI) and chymotrypsin inhibitor (CI). In many organisms. light is a crucial environmental signal influencing natural physiological and developmental processes. There are different effects on the phenotypic and physiological characteristics even if the same plant species with the long-term growth in different light condition. In order to understand the impact of sunlight on the constitutive defenses of Larix gmelinii, sunlight control treatments with three intensities: 100% of natural sunlight (NS), about 50% and 25% of natural sunlight (NS/2, NS/4) were conducted to simulate the light conditions of forest edge, forest gap and understory where the Larix gmelinii grows. Sunlight control was achieved by erecting shade shelters of woven black nylon netting. The activities of the primary defense proteins in the needles of Larix gmelinii seedlings under different light treatments were measured. The activities of POD, SOD, PAL, PPO and CI under 50% and 25% of natural sunlight conditions were significantly (P<0.05) higher than those under 100% of natural sunlight conditions both in June, July and August. The activities of POD and CI were greatest under 50% of natural sunlight conditions while the activities of SOD, PAL and PPO were greatest under 25% of natural sunlight conditions. But the CAT activities were significantly (P<0.05) decreased under the shade conditions. The activities of TI were significantly (P<0.05) changed under different light treatments, but the variation was not regular. The activities of the primary defense proteins showed significant (P<0.05) differences under the same light intensity within the three-month experiment. The activities of POD and CAT in July and August, were significantly (P<0.05) higher than in June. The activities of SOD, PAL, PPO and CI were highest in June. Our results suggested that different light intensities can significantly affect the activities of the primary defense proteins. SOD, CAT, PAL, PPO and POD are the key enzymes in the production of the secondary metabolites of Larix gmelinii. The observed changes indicate the plant defense system has been changed. Our findings therefore suggeste that Larix gmelinii could endure a low light intensity environment and a suitable light deficit might be a feasible means to increase the defensive protein activities of Larix gmelinii. Furthermore, Larix gmelinii could likely be used in forestry systems designed to limit light with increasing forest canopy density or in the schattenseite. Studying the correlation between plant defense and environmental factors may yield both theoretical and practical implications for future studies as well as field applications.