Abstract: In view of the problem that the deposit with density larger than the heavy liquid is easily to be drained by centrifugate,thus,it can affect the experiment results in small floating and sinking test.Then by adding magnetite powder during the experiment,this study succeed in improving accuracy of float and sink test.The relationship between different dosage of magnetite and different ash content of coal samples was discussed.The results showed that increase of magnetite dosage can dramatically augment the viscosity of suspension liquid,which apparently avoids the aforementioned drainage by centrifugate,and the magnetite powder can be recovered easily,hence a more accurate weighted ash can be obtained.It also proved that productivities of each density class extracted from this innovative method are more effective than traditional ones.