This article describes an online detection system for measuring the geometrical parameters of the microtarget in inertial confinement fusion (ICF). The measurement can be done during the microtarget's assembly process and then the message, according to the online detection results, can be sent to the assembly robots as the adjustment order. This detection system consists of three parts: a laser confocal system, a machine-vision system, and a motion-control system. During the real-time monitoring in the assembly process, the machine-vision system detects the position information of each part and the laser confocal system detects the reference points which have higher-precision requirements. The laser confocal system and the machine-vision system can be operated by the motion-control system so as to achieve the best measurement effect. The test data got by the two systems are then unified in the calibrated coordinate system. Using various fitting algorithms, the authors can find the parameter status, make the error analysis, or evaluate the tested beeline, round, sphere, and so on. With the error analysis of the motion-control system and the experiments, the measurement results is less than 3 mu m, which can meet the requirement of the assembly system. So this online detection system is feasible.