Abstract: In order to solve the technical difficulties in the efficient downhole gas-liquid separation of the oil wells with high gas/liquid ratio, this paper designed and developed a new type of three-stage efficient downhole gas-liquid separator suitable for multi-pattern fluid and established a set of gas-liquid two-phase experimental evaluation system suitable for multiple flow patterns. In the experiment, the liquid flow and void fraction at the inlet of the separator are 5~300 m³/d and 5.0%~94.1%, respectively. The experimental study results show that the liquid level height between inner cylinder and outer cylinder is a key index to ensure the efficient separation in the separator. If the liquid level height is controlled below the reasonable critical liquid level, gas-liquid mixture of wide flow, high void fraction and multiple flow patterns, especially oscillating flow pattern can be separated efficiently. What’s more, this separator is verified to have wider self adaptability. At constant inlet opening and outlet opening, efficient separation can be realized when the inlet void fraction varies in the range of 26.7%~94.1%. The technology realizes the efficient gas-liquid separation under the working conditions of downhole small space, high void fraction, wide flow and multiple flow patterns and provides the effective solution for the smooth operation of electric pump discharge/lifting in the wells with high void fraction in offshore oil and gas fields.