To investigate the spatiotemporal dynamics of DNA damage and repair after particle irradiation,an online live cell imaging system has been established based on the microbeam facility at Institute of Modern Physics(IMP).In the experiment of live HT1080 cells(RFP tagged XRCC1)imaging,XRCC1 foci were formed without irradiation,which were mainly caused by the phototoxicity of fluorescence imaging XRCC1(X-ray repair cross complementing group 1)is a base excision repair protein which is required for DNA single-strand break repair in human cells[1]and XRCC1 is really a sensitive marker to characterize the phototoxicity of the live cell imaging system.