Abstract: Investigation on an accident of tubing collapse and fracture happening during injecting diluted oilinto casing for exploitation in one dense oil well was done.It was considered that the tubing was in accordance withthe standard requirement based on material inspection,dimension measure and tubing collapse test.It was found outthat the tubing collapse was not related to mechanical damage because there was no mechanical damage on the tubingsurface before collapse by inspecting the collapsed tubing.It was concluded that the tubing was collapsed first,andthen fractured by reasoning and analysis.It was found that the cause of the tubing collapse and fracture was the loadborne on tubing exceeding its yield strength according to mechanics analysis and calculation.