Abstract: Due to the factors such as drilling fluid invasion, reservoir permeability, pump efficiency and other factors,pure undisturbed formation fluid samplescannot be obtained during the test operation of the cable formation, which leads to great difficulty in judge of reservoir fluid properties. Based on the traditional fast water analysis technology, in this paper, it is proposedthat formic acid can be used as the characteristic ionto calculate the purity of the sample, and to determine the fluid properties of the sample. At the same time, it is proposed the relationship between sample purity and resistivity change rate can be obtained by using the resistivity calculation method, and then the optimal time node for the cable formation testing is proposed. The practical application of dozens of wells at sea shows that the technology can not only provide an important basis for the field operation decision, but also greatly reduce the cost of exploration and reduce the risk of operation.