The invention provides a method for preparing tantalum powder with the high specific volume. The method includes the following steps that 1, tantalum pentoxide and a carbonaceous reducing agent are taken by weight, dissolved in deionized water and stirred to be even to obtain mixed liquid; the prepared mixed liquid is put into a pressure filter and subjected to filter pressing in a pressing mode to obtain a filter cake; the filter cake is heated, dried, smashed and sieved to obtain precursor powder with tantalum oxide and carbon sources finally; 2, the precursor powder obtained in the step 1 is placed into a vacuum furnace and subjected to carbon reduction under the vacuum to obtain tantalum oxide powder with the low oxygen content; 3, low oxidation state powder of tantalum and mental magnesium powder are mixed and then subjected to a reduction reaction in the atmosphere of inert gas or vacuum to obtain high-oxygen tantalum powder; 4, the high-oxygen tantalum powder and magnesium powder are mixed and deoxidized for the second time to obtain tantalum metal powder suitable for a capacitor; after the step that the tantalum powder is deoxidized through magnesium, magnesium oxide and residual magnesium metal are removed from a reaction product.