The present invention relates to a pharmaceutical composition which is for preventing or treating brain cancer, a preparation method thereof, and the pharmaceutical composition for inhibiting brain cancer metastasis. The pharmaceutical composition is a pharmaceutical composition comprising a polymorph of tetraarsenic oxide (As4O6) as an active ingredient, which contains more than 99% of a polymorph (a) of tetraarsenic oxide (As4O6) having the following characteristics (i) to (iii), wherein (i) unit cell constant: alpha=beta=gamma=90 degrees, (ii) As-O bond length: (iii) O-As-O bond angle: 98.36 degrees. The composition according to the present invention exhibits excellent cancer cell proliferation inhibition and metastasis inhibition effects, and thus may be usefully used as an anti-canceragent.