The invention discloses a preparation method of big-specific-surface IrO2. The method comprises the following steps of (1) ultrasound dissolution; (2) spray drying: performing spray drying treatment on the solution obtained in the step (1) to obtain micrometer grade spherical ammonium chloroiridate powder; (3) microwave roasting; (4) washing and drying: performing ultrasonic dispersion washing anddrying on the powder obtained in the step (3); then, obtaining a big-specific-surface IrO2 powder product. The IrO2 powder is prepared by using an ultrasonic dissolution, spray drying and microwave roasting method; the ammonium chloroiridate powder is subjected to ultrasonic treatment; the dissolution is enhanced; the ammonium chloroiridate solution is subjected to spray drying to obtain the micrometer stage spherical ammonium chloroiridate powder with good flowing dispersion performance; then, through the microwave roasting, the IrO2 powder is obtained. The technical flow process is simple;the implementation is easy; the purity of the IrO2 powder is greater than 99.95 percent and is similar to the spherical shape; the flowing dispersion performance is good; the particle diameter is 0.5to 5mu m; the specific surface surface is greater than 150 m<2>/g.