An optical instrument for rapid detect ion of bacteria from natural water is presented in this paper,which is based on the principle of adenosine triphosphate (ATP) biolumines cence reaction.The E.coli and real samples from Taihu Lake were tested by the optical instrument,and results were compared with TBC results provided by plate counting.The instrument integrated with high-precision AD converter and a small photomultiplier tube (PMT) module is used to improve the optical detection sensitivity.Combined with homemade ATP bioluminescence biosensors,this instrument can be used for rapid de tection of bacteria.An calibration curve is obtained from the bioluminescence in tensity for E.coli in the range of 4.6×101CFU/mL－ 4.67×107CFU/mL with a good orrelation coefficient of 0.9515.Three methods of sample treatment were compared in order to test bacteria from natural water.Then,23samples and 40s amples were collected and tested in summer and winter,respectively,from Taihu Lake,Wuxi of China.In summer coll ection,a linear relationship between RLU and TBC provided by plate counting is obtained with correlation coe fficient of 0.9169.In winter collection,the results of TBC by K2010show that 40samples are all mesotrop hic,consistent with plate counting method,which indicates that 38samples are mesotrophic and 2samples are oligotrophic.In summary, this system can complete a process of detection in a few minutes,and with high s ensitivity and reproducibility,so it has potential in many fields,suc h as food safety,environmental monitoring,and water pollution forecast.