The invention discloses a high-mud-resistance polycarboxylic acid water reducing agent and a preparation method thereof. The weight average molecular weight is 40000-90000. The high-mud-resistance polycarboxylic acid water reducing agent is prepared by carrying out dissolution, free radical polymerization and neutralization on the following components in parts by weight: 1900-2100 parts of isopentene alcohol polyoxyethylene ether with the molecular weight of 2000-3000,, 110-150 parts of acrylic acid, 10-20 parts of tripropenyl phosphite, 20-50 parts of dimethylaminopropyl acrylamide, 15-40 parts of initiator, 8-30 parts of chain-transfer agent, 2-10 parts of composite reducer, 100-150 parts of liquid caustic soda and 3000-3100 parts of deionized water. When the sandstone material has higher mud content, the high-mud-resistance polycarboxylic acid water reducing agent still has equivalent concrete water reducing rate to the common polycarboxylic acid under the condition of lowering the doping amount in the same experimental environment, has better concrete slump retaining effects, and can enhance the concrete strength by 1-2 levels. The high-mud-resistance polycarboxylic acid water reducing agent has the same effect when being compounded with other polycarboxylic acid water reducing agents.