Abstract: Discussed from the mechanics point of view are the reasons for the frequent failures of the pressure spring in the paraffin scraping mechanism for floating roof tank. The reasons are the following: ①The pressure spring is not installed as designed, resulting in an actual bending deflection of over 88% of the designed value, and a disallowable normal stress at the foot of spring where exists the maximum bending moment; concequently, the centralized stress on the pressure spring gives rise to mi-crocracks.②Due to the resistance and impact from the welds of shell plates and the projection welding and residues left during construction, the pressure spring may crack with fatigue after a long time period of service, specific repairing programs are given accordingly.