The invention relates to brucite fiber paste used for toughening cement-based materials and a preparation method thereof. The brucite fiber paste comprises natural brucite fibers, water reducing agents and water in a weight ratio of 1:(0.2-1.2):(0.15-0.75), wherein, the natural brucite fiber is of grade 6 or 7. The preparation method is characterized by adding the above raw materials into a forcedmechanical mixer to be mixed and stirred for 5-15min and then placing the mixture for more than 24h in an airtight mode. The brucite fiber paste is used in cement concrete as a toughening material. The usage of the brucite fiber paste in the cement concrete is 5-15% of that of the cement. The brucite fiber paste and other raw materials are simultaneously added into the mixer while the cement concrete is blended, and are mixed for 2-4min. The brucite fiber paste is used for the concrete, enjoys good fiber split property, good construction technological property and low cost, is easily dispersed in the concrete, can avoid dust fly while the concrete is blended and ensures obvious fiber toughening effect.