Abstract: The design and structure specificities of loose part measurement system (LPMS) of VVER in Tianwan Nuclear Power Plant, as well as the differences of LPMS with the item NRC RG1.133, are introduced. The difficulties and the negative influence are analyzed based on these differences and the specificities of VVER, when the improvement is conducted. In order to implement the function like what NRC RG1.133 requested, additional tests of LPMS were carried out during the commissioning of Unit 3 in Tianwan Nuclear Power Plant. The signal response of the sensors related to the reactor pressure vessel (RPV) was detected successfully in the tests, and it was proved that the arrangement of the LPMS sensors could meet the design requirement of NRC RG1.133.