This paper proposed a passive composite hygroscopic material to control indoor humidity based on field experiment. A series of aggregates including wood fibre, sepiolite, expanded perlite and diatom ooze were mixed at different ratios to make an optimal composite hygroscopic material for further experiment. Two testing rooms，which were attached with conventional cement mortar（Room 1） and composite hygroscopic material（Room 2），respectively，were built to conduct comparison experiments（mainly testing temperature and humidity） in January and April in 2017 in Changsha, Hunan. The hygroscopic performances were evaluated by the practical Moisture Buffer Value，and performances of indoor environment mold inhibition of two rooms were also analyzed. The results showed that in the two tested seasons，Room 2 outperformed in hygroscopicity than Room 1, where the relative humidity was lower by 6%-14.3% and moisture content was lower by 0.3-3.1g/kg·a. In the cold January， the risk of mold growth in Room 1 was about 4 times higher than that in Room 2，while in humid April，the risk rate of mold growth in Room 2 was only about 2%（6.5 h in monitored 288 h），which was obviously lower than that in Room 1. The newly studied composite hygroscopic material was found to be efficient and practical in controlling indoor humidity.