The thermal and dimensional stability of epoxy resin(EP)in-situ modified by cyanate ester(CE)and polydimethylsiloxane (PDMS) are investigated by means of experiments and numerical simulation. Thermal gravimetric analysis(TGA)and differential scanning calorimeter(DSC)are used to analyze the heat resistance of the modified EP. The dimensional stability is characterized by the volume shrinkage of the series PDMS/CE/EP obtained by the density method. The chemical structure of the PDMS/CE/EP is analyzed by Fourier transform infrared spectroscopy(FTIR). The results of TGA and DSC indicate that the thermal stability of PDMS/CE/EP decreases firstly and then increases with the increase in the amount of CE. The addition of PDMS shows a slight effect on the thermal stability. The 40％ CE makes the blending system exhibit the lowest initial decomposition temperature,which reduces by 15.5％ and 40.8％ compared with pure EP and CE,respectively. The FTIR results suggested that the influence of CE on the thermal stability of the modified EP is mainly ascribed to the generation of oxazolidinone ring with low thermal stability and the increase in the triazine ring with high thermal stability. The volume shrinkage measurement results show that the introduction of CE and PDMS are both beneficial to the improvement of the dimensional stability of the blending systems. The in-situ addition of 80％ CE shows the lowest volume shrinkage of 6.11％. The thermal stress distribution of PDMS/CE/EP generated during the solidification process is simulated by the finite element analysis. The results suggested that the introduction of 80％ CE into EP results in the lowest thermal stress in the blending system,which indicates that the system has the lowest volume shrinkage,which agrees well with the experimental results.