An electrochemiluminescence immunoassay system and a flow-through cell unit (1) thereof. The flow-through cell unit (1) comprises a working electrode (12), a counter electrode (11) and a reference electrode (13), wherein the working electrode (12) and the counter electrode (11) are vertically provided, and there is a liquid flow path between the two electrodes. When an electrochemical reaction occurs, and when reactants are evenly distributed on the working electrode (12) for a test. After the flow-through cell is cleaned, the liquid flow path is unblocked, so that the cleaning effect is better. A porous structure is provided on a connection surface where the reference electrode (13) is in communication with the liquid flow path, and the porous structure makes the reference electrode (13) have a good electrical conductivity, and prevents the reference electrode (13) from early ageing, and improves the durability of the flow-through cell unit (1).