The invention discloses an electrochemical sensor used for instantly monitoring and detecting the water biotoxicity. The electrochemical sensor comprises a work electrode, a counter electrode, a reference electrode and a electrolytic cell; the work electrode is a polymer fixed microbial electrode, and the polymer fixed microbial electrode comprises an electrode and a polymer and bacteria mixed layer coated outside the electrode; and the material of the above polymer is gelatin or chitosan, the above bacteria comprise Escherichia coli and / or yeast, and the electrode is a glassy carbon electrode, a gold electrode, a platinum electrode or a boron-doped diamond film electrode. The invention also discloses an apparatus of the electrochemical sensor used for instantly monitoring and detecting the water biotoxicity. The electrochemical sensor can instantly monitor the water biotoxicity change and detect the water biotoxicity value, can realize instant, online and continuous detection, and has the characteristics of high analysis sensitivity, low cost, simple operation, portability and the like.