Abstract: The capric-lauric fatty acid (CA-LA)/diatomite filter aid form-stable phase change composite was prepared by vacuum impregnation method. The prepared form-stable PCM was characterized by diffusion-effusion circle test, FTIR, SEM, BET, DSC and TG technique in the form-stability, absorption mass, thermo-physical properties, thermal stability and heat storage/release performance. The results show that the adsorption capacity of the diatomite filter aid to the CA-LA eutectic is about 49.88wt% without the effusion circle in the composite, and the phase change temperature and latent heat is 21.8℃ and 75.45 J/g, respectively. The CA-LA/diatomite form-stable composite has good thermal stability and reliability which proved by TG and melt/freezing cycling test. After 100 times thermal cycling, the phase change temperature and latent heat of the CA-LA/diatomite form-stable composite change a little. It is also revealed that the CA-LA/diatomite form-stable composite can charge and discharge latent heat so that the environment temperature can be adjusted and regulated in some degree. The prepared CA-LA/diatomite filter aid form-stable composite is promising to be applied in the building energy conservation and heat storage applications.