A visual detection method of relative liquid level without camera calibration and liquid level gauge and closed-loop control system are proposed for the robotic pouring task related to multiple types of transparent containers and liquids. Firstly, the characteristics of the service robots′ liquid pouring tasks are analyzed and the target detection method in computer vision field is deployed to detect liquids and containers simultaneously. The relative liquid level height is obtained by calculating the height-proportion between the detected container and detected liquid, which avoids the complicated hand-eye calibration processes during the measurement of liquid level′s absolute height. Secondly, the proposed liquid level detection method is geometrically modeled and analyzed by applying the pinhole imaging model, and the laws on measurement errors of relative liquid level height are deduced in typical cases. Thirdly, the images of various liquids and containers are collected to train YOLOv5s, which is deployed to detect target object and obtain the relative height of liquid level. The test results verify the effectiveness of proposed method. The paired average precision of new method is 86. 7% for the new types of liquids in new shaped containers that do not appear in the training set. Finally, to avoid solving or estimating corresponding Jacobian matrices in visual servo theory, the detected liquid level height is combined with PD control to get a closed-loop control system. Several kinds of liquid pouring tasks on both manipulator platforms are successful with the same parameters in PD controller, which prove the effectiveness and robustness of proposed closed-loop control method.