Abstract: The transient characteristics of containment loop heat pipe cooling system are studied,and the characteristics of natural circulation flow rate,containment pressure and flow instability of the heat pipe cooling systems are analyzed by liquid head start-up based on the experimental data.The results show that the start-up of containment cooling system works well.In accident conditions,such as LOCA or MSLB,the heat of the containment vessel can be successfully transferred out.In the start-up stage,containment pressure drops rapidly,which is good for containment pressure inhibition.Flow instability of the containment cooling system happens under some conditions,which relevant to cooling-tank temperature,containment pressure and input heat power.Flow instability has no obvious effect on the trend of pressure in the containment.