The invention provides a preparation method of a microalga-based biochar material for heavy metal ion adsorption. The preparation method comprises the following steps that 1, microalgae and water are added into a hydrothermal reaction kettle to prepare microalga suspension liquid; 2, organic acid is added into the microalga suspension liquid, and after the hydrothermal reaction kettle is sealed, protective gas is introduced into the hydrothermal reaction kettle to replace air in the hydrothermal reaction kettle; 3, the hydrothermal reaction kettle in the step 2 is placed in a drying oven for a carbonation reaction, and the microalga-based biochar material is obtained; 4, activating treatment is performed on the microalga-based biochar material, and the activated microalga-based biochar material is obtained. According to the preparation method, biochar is prepared through hydrothermal carbonization by taking the nitrogen-containing microalgae as the raw material, and then activated modification is performed on the biochar through a KOH activating method and a NH3 activating method separately by taking the biochar as a precursor; the activated microalga-based biochar material obtained through activated modification has micro-mesoporous structures, the large specific surface area and the large pore volume, and the adsorption capacity of porous charcoal is greatly improved.