Abstract: Open-field cultivation has been the main mode of citrus cultivation in China, with major blooming peak in spring and citrus fruits only shipped to markets in October through December. Under protected cultivation, it was possible to force the flowering stage of citrus to an advanced date via temperature control for earlier shipments of fruits to markets. In north China, early autumn low temperatures in solar greenhouses have been used to induce early citrus flower bud differentiation. Induction of flowers by the warming effect with plastic film covers forced sprouting. While the role of the low temperature in flowering has been well documented, flower bud differentiation competence during winter under greenhouse conditions has remained unconfirmed. To determine suitable heating time for citrus flower bud morphological differentiation in solar greenhouse, 4-year-old of "Shatangju" tangerine (Citrus reticulata "Shatangju") seedlings with stock of trifoliate orange (Poncirus trifoliate Raf.) were selected. The response of the "Shatangju" tangerine flower to low temperature in solar greenhouse was studied in an in vitro culture. The results showed that some axillary buds differentiated into flower buds after treated with low temperature (15 ℃ at day for 12 h, 10 ℃ at night for 12 h) for 720 hours. Floral ratio apparently decreased while germination ratio sharply increased after 960 hours under low temperature. Floral ratio was negatively correlated with germination ratio. Under solar greenhouse conditions, autumn shoots of "Shatangju" tangerine were ripe enough to form flower buds in early November. Floral ratio apparently increased during the whole of winter, which showed order of average floral ratio: February > January > December. When accumulated time of low temperature 5~20 ℃ reached 1 309 hours, floral ratio stabilized at 40.51%. A significantly negative correlation was noted between leafy to leafless flower ratio and accumulated hours of <5 ℃. Under low temperature treatment, C/N ratio was positively correlated with floral ratio while proline content showed a highly significant positive correlation with floral ratio. The response of the change in C/N ratio and proline content of "Shatangju" tangerine was positive to flower induction. In conclusion, change in C/N ratio and shoot proline content were good indicators for heat treatment of citrus to force flower bud germination.