Abstract: The virtual instrument based on LabVIEW 7 Express for determinating the dissociation constant of monoatomic weak acid was developed by connecting the general conductivity meter and temperature sensor with computer by homemade adjusting circuit.The virtual instrument not only has advantages of friend interface,easy operation,easy expandability of function,etc,but also has the function of real-time temperature compensation.The automatical acquiring,real-time displaying,processing and saving of experimental data are realized.By this method,the complexity of manual operating and personal error are avoided;the accuracy and repeatability of testing result are improved;the function of traditional instrument is also expanded.The application results are satisfactory.