Abstract: The slight boiling operation of the primary loop coolant is one of the key ways of nuclear heating reactor co-generation of electricity and heat. In the case of the slight boiling operation condition, two-phase flow system stability becomes worse and more complicate. The influence of gas space on two-phase flow stability was testified experimentally, and a new method to restrain system instability by changing gas space structure was proposed based on the experimental results. The inserting of plates with small hole can effectively restrain the flow oscillation amplitude and improve the boundary of the two phase flow stability on some extent.