The invention relates to a microminiature activating device for thermal batteries. The microminiature activating device comprises piezoelectric ceramic plates, electrodes, an electric ignition head, an insulating layer, an insulated conductor, a gravitational block, ignition bars, a monomer cell, a shell body, cell poles, conducting wires and a piezoelectric ceramic reactor, wherein the shell body is divided into an upper layer and a lower layer, the monomer cell is arranged inside the upper layer, the cell poles are upwards arranged at the top end of the monomer cell, the ignition bars are arranged on the external wall and bottom end of the monomer cell, and the gravitational block, the insulating layer, the electrodes and the piezoelectric ceramic plates are sequentially arranged in the lower layer from top to bottom; the gravitational block is cylindrical, a circular through hole is formed in the middle of the gravitational block, the electric ignition head is arranged inside the circular through hole, an interlayer is arranged between the upper layer and the lower layer, and an opening is formed in the middle of the interlayer. According to the microminiature activating device for the thermal batteries, the piezoelectric ceramic plates are adopted as an activating energy source, the level of integration is high, the reliability of activation reaches 99.5% or more, and the reliability is good; compared with the existing activation modes, the microminiature activating device has the advantages of low space usage and low manufacturing cost.