Abstract: In the diffraction limit storage ring, the size of the vacuum chamber was greatly reduced compared with the third-generation synchronous light source due to the high-gradient small-aperture magnet, and the impedance of ring is significantly increased. Therefore, it is necessary to accurately evaluate and optimize model of the impedance of the vacuum elements in the design stage. In the impedance budget, the resistance wall impedance accounts for a high proportion of the whole ring impedance, and a sharp resonance peak will appear when the frequency is close to 0, which has a great impact on the beam motion. In the RF system, the high-order mode of the cavity is an important source of the narrow-band impedance. Therefore, in the accelerator design stage, it is necessary to select the resistance wall of appropriate materials, accurately model of the RF cavity and the third harmonic cavity, calculate the impedance, and evaluate instability that may be caused. In this paper, the multi-layer resistance wall impedance of the medium energy diffraction limit storage ring was calculated and the optimized material of resistance wall was chosen for the diffraction limit storage ring. A vacuum chamber with distributed exhaust is required in the storage ring, and the thickness of copper coating of 1 μm film is the optimized material. For special vacuum chamber components, the internal structure is complex and it is impossible or difficult to implement NEG coating, or the centralized arrangement of air pump can meet the vacuum requirements, the stainless-steel internal coating with 5 μm copper film is considered. The impedance models of RF cavity and the third harmonic cavity were established and optimized by using numerical simulation program. The fluted beam tube can effectively transmit the high-order modes generated in the RF cavity and the third harmonic cavity to ensure the stability of the beam. The collective effects were caused by resistive wall impedance, the high order modes of RF cavity and the third harmonic cavity were analyzed and summarized. In the medium energy diffraction limit storage ring, the transverse impedance of the resistance wall will cause the transverse coupled multi beam instability, the impedance of the RF cavity and the third harmonic cavity will not cause the longitudinally coupled multi beam instability, and will not become a limiting factor for the improvement of the beam intensity in the subsequent light source upgraded. Meanwhile, effective control measures are put forward to solve the problems of beam instability. For transverse coupling multi beam instability, positive chromatic can be effectively suppressed.