The invention discloses a trapping method for tar in coal gas. The trapping method comprises steps as follows: gas-gas heat exchange: low-temperature gas exchanges heat with high-temperature coal gas, heat of the high-temperature gas is recovered, and the use amount of spray tar is reduced; trapping: low-temperature tar is sprayed on the high-temperature coal gas subjected to heat exchange, so that tar gas in the coal gas is condensed, separated out and recovered through trapping; defogging: tar fog drops about to escape are recovered gradually by use of a two-stage defogger. With the adoption of the tar trapping method, tar can be removed from the coal gas repeatedly, that is, the better tar recovery effect is realized in a tar washing manner. Compared with an existing technology for trapping tar through washing, firstly, the tar trapping method cannot produce a large amount of refractory wastewater, so that the investment, production and operation costs of wastewater treatment equipment are reduced; secondly, the trapping rate is high and secondary investment of an electrical tar precipitator is reduced. Besides, the invention further provides a trapping system for the tar in the coal gas.