In order to improve the efficiency of crude oil production, the water injection production and horizontal well production are widely used in the oil fields. This brings higher requirements for online water holdup measurement of oilwater twophase in downhole. There are two main challenges: the one is to guarantee the high detection accuracy of sensor under the condition of high water holdup and low water holdup, and the other is the small structure of single sensor to arrange multiple sensors conveniently to form detection array on the cross section of the oil well, so as to improve the spatial resolution of the measurement by providing the distribution information about water hold up on the crosssection of horizontal wells or highly deviated wells. Hence, this work presents the water holdup detection method of oilwater twophase flow based on coplanar microstrip transmission line. Conformal transformation method is utilized to analyze the relationship between the structure parameters and material parameters of coplanar microstrip transmission line sensor and electromagnetic wave propagation characteristics. The doublesided and Sshaped wiring sheet structure is used not only to miniaturize the sensor but also improve the dynamic range of detection Numerical simulation and experimental results show that the phase shifts of the detected signal are approximately linear with the water holdup of the measured fluid. The resolution is less than 3% in the whole range of water holdup from 0% to 100%. This method overcomes the disadvantages of capacitance method and conductance method, and is suitable for low water holdup and high water holdup, respectively.