Abstract: In order to remove decay heat safely from the drain tank, instability research experiment was carried out in thimble tube-type natural circulation loop. The observed phenomena and collected data were analyzed. The results show that the fluid flowing in the center pipe is preheated obviously. Several wave peaks appear when the liquid level goes up and down at the outlet of the heat transfer tube. The first one is called for main peak. Others are called for subordinate peaks. Duration time of subordinate peaks is often longer than that of main peak. Oscillation frequency of temperature at the inlet is higher than those of other parameters. Water in the tank is subcooled when the oscillations occur. The period of oscillation is not obvious. There exists coupling between low frequency oscillations. The positive maximum deviation between the period of oscillation and boiling delay time is 71%, and the negative maximum deviation is 37%.