Moss bogs dominated by bryophytes play important roles in the maintenance of biodiversity and water resource in karst regions. There is limited knowledge about the effects of artificial forests on the ecological functions of moss bog ecosystems. To understand the effects of artificial Cryptomeria fortunei forests on the plant community and water content of a Polytrichum commune bog, we chose a typical P. commune bog in a karst region in southwestern Guizhou Province to conduct field surveys. We found that, 1) the artificial C. fortunei forest negatively affected P. commune communities. The cover, average height, and species richness of the P. commune community decreased significantly with increasing density of C. fortunei. 2) Surface soil contributed most to the water content of the bog ecosystem. The C. fortunei forest significantly improved water content in the surface soil in the dry season. High-density C. fortunei forest significantly reduced the water content in the surface soil in the rainy season. The C. fortunei forest did not affect the water content of the P. commune in the dry season. Low-density C. fortunei forest improved the water content of the P. commune in the rainy season. 3) There were seasonal variations in the effects of C. fortunei forest on the water content of P. commune bog ecosystems, i.e., a negative relationship in the rainy season was noted, but this shifted to a positive one in the dry season. 4) The C. fortunei forest was beneficial for maintaining the stability of water content of the bog ecosystems, especially when the density of C. fortunei was not too high. This suggests that the effects of artificial C. fortunei forest on P. commune bog were nonlinear. Water content of the P. commune bog can improve if the density of C. fortunei is reasonable.