The invention provides a filter aid for refining thick liquid by a using a Bayer process and relates to the filter aid for refining the thick liquid by using the Bayer process in the production process of aluminum oxide. The filter aid for refining the thick liquid by using the Bayer process is characterized in that the filter aid is a mixture of coal ash and lime milk. According to the filter aid provided by the invention, the filter aid is the mixture of the coal ash and the lime milk and is added into the sodium aluminate thick liquid in the Bayer process to be subjected to thick liquid refining, the consumption of lime and the loss of the aluminum oxide in the thick liquid refining process of the sodium aluminate can be reduced, the concentration dilution of a system is reduced, and the emission load of waste of a thermoelectric boiler is reduced.