In this paper, the optothermal microactuation technology and optothermal microactuator (OTMA) suitable for water or other liquids are proposed and developed. The model of optothermal expansion and temperature rise distribution is established, and simulation on a 1080 μm long OTMA is conducted, revealing the feasibility of optothermal microactuation technology in water. The optothermal microactuation experiment of a symmetrical OTMA is carried out in water under the irradiation of a laser with a wavelength of 520 nm and adjustable power, revealing that the optothermal deflection increases with the increase of the laser power. Another experiment is carried out under the irradiation of a laser pulse with a wavelength of 520 nm, effective power of 4 mW, and an adjustable frequency, demonstrating that the symmetric OTMA has a good dynamic response under the laser irradiation. The amplitude of the actuating (deflection) amount varies between 2.6 μm and 3.7 μm when irradiated by the laser pulse with a frequency of 0.9 Hz~16.4 Hz, and it decreases with the increase of the frequency of the laser pulse. The theoretical research and experimental curve trend reveals that it is completely feasible to obtain greater deflection and higher frequency optothermal microactuation in water by appropriately increasing the laser power and laser pulse frequency. This research provides new methods and approaches for the application of micro-opto-electromechanical systems and micro-nano technology.