Walnut (Juglans regia) has been regarded as an allelopathic tree species for a long time. In the management of agroforestry systems which are able to take good advantage of land resources, the allelopathic effect of walnut on interplanting crops elicits increased concerns. However, the environmental factors may alter markedly the allelopathic effect of a plant. Gas chromatography-mass spectrometry (GC-MS) was used in the present study to identify and compare chemicals in walnut leaf litter collected from two different habitats, i.e., Qinba mountainous area (QMA) and high-mountain gorge area of West Sichuan (HGWS). A container experiment was performed to investigate the effects of the walnut leaf litter from these two regions on growth and photosynthetic characteristics of pakchoi (Brassica chinensis) during the early decomposing stage of the litter. Walnut leaf litter was applied at the rate of 0, 30, 60, 120, and 180 g of leaf litter per pot, resulting in five treatments: F0 (no leaf litter), F1, F2, F3, and F4. Each container was filled with 8 kg of soil mixed with corresponding amount of walnut leaf litter, and the pakchoi seeds were sowed. The growth indices of pakchoi were determined 20, 45, and 75 days after sowing, and the photosynthetic parameters were determined when the 3^rd euphylla had emerged in almost all pakchoi seedlings exposed to treatment F4. The results were as follows: (1) Twenty-one compounds with a relative content no less than 1% were identified by GC-MS, which were classified into eight categories. Of them, 12 compounds with a relative content no less than 1% from five categories were identified from the QMA walnut leaf litter, representing 51.46% of the total chemicals, and 15 compounds with a relative content no less than 1% from seven categories were identified from the HGWS walnut leaf litter, representing 56.03% of the total chemicals. Some chemicals which were frequently reported to be allelochemicals, such as calarene, hinesol, beta-eudesmol, hexahydrofarnesylacetone, linolic acid, stearic acid, 1-octadecanol, gamma-sitosterol, and beta-sitostenone were detected in higher quantities in the HGWS walnut leaf litter compared to the relative content; while some other chemicals, such as squalene, delta-tocopherol, and loliolide were higher in the QMA walnut leaf litter. (2) The biomass of pakchoi significantly decreased in the treatments with walnut leaf litter regardless of the amount at 20 and 45 days after sowing; however, at 75 days after sowing, the inhibition effect of the litter decreased. (3) At the early decomposing stage, the contents of all photosynthetic pigments decreased progressively as the amount of leaf litter increased; simultaneously, the net photosynthetic rate (P_n), the stomatal conductance (G_s), and the transpiration rate (T_r) were significantly inhibited by the decomposing walnut leaf litter. (4) On the whole, the allelopathic effect of the HGWS walnut leaf litter was stronger than that of the QMA walnut leaf litter. It is hypothesized that walnut trees accumulate more allelochemicals (ketones, alkenes, alcohols, steroids, etc.) in the high-mountain gorge area of West Sichuan because of the more stressful environmental conditions, thereby exerting stronger allelopathic effect via decomposition of its leaf litter.