The invention provides a micro pressure shear experimental device and a testing method of mechanical property of a fusion welding microcell. The experimental device comprises a base, a lower die, a guide pressure head, a pressure head and a pressure rod, wherein the center of the base is provided with a concave table; the upper part of the base is provided with two guide posts, a groove is formed between the two guide posts, and the groove is communicated with a concave table; the lower die is installed in the concave table, and the top surface of the lower die is in flush with the bottom of the groove; a cylindrical hole is arranged between the two guide posts, and the guide pressure head is installed in the cylindrical hole; the center of the guide pressure head is provided with a guide hole; and the pressure head and the pressure rod are installed in the guide hole. The micro pressure shear experimental device provided by the invention is reasonable in design, can guarantee the loading precision and accuracy, is reliable in data collection, and is accurate in obtained load-displacement curve of each cell; and through testing, shear strength and an engineering shear stress-strain curve of a welding joint parent metal joint are obtained, and can be expanded to each cell of the joint, thus a microcell heterogenous mechanical property of the joint can be obtained.