Two-phase closed thermosyphons (TPCTs) have been widely used in solar energy utilization. The work presents an optimization of the TPCT used in cooling foundation in permafrost regions. A series of experiments under negative temperature conditions were conducted to investigate the effect of length ratios (LRs, length ratio of the evaporator section to the condenser section) on the cooling performance of the TPCT. Experimental results show that there exists a best LR for each angle, and it varies with the inclination angle. As an example, for the TPCT with 5.0-cm inner diameter, the maximum efficiency can be decreased by 17.3% for the TPCT with the LR of 1.00 compared with the TPCT with the best LR of 1.65. Under the best LR, the efficiency can be raised by 30.2% for the TPCT with the inclination angle of 20 degrees compared with that of 90 degrees. Besides, the best LR also changes with the pipe inner diameter, e.g. the best LR varies from 1.65 to 1.27 when the inner diameter changed from 5.0 cm to 8.1 cm. The results suggest that the equivalent length of the evaporator section should be appropriately lengthened to enhance the cooling performance of TPCTs in permafrost engineering.