A bisbiphenylacylphosphine oxide of formula (I) and its preparation method are provided. The formula of —Ar— is First, 10-chloro-9,10-dihydro-9-oxa-10-phosphaphenanthrene (CDOP) is prepared by using 2-phenylphenol, and then is esterified to synthesize 6-methoxy-(6H)-dibenz[c,e][1,2]oxa-phosphorin (MDOP). Next, acid chloride compounds are added for performing the Arbuzov reaction to synthesize bisbiphenylacylphosphine oxide. CDOP is hydrolyzed to be derived into 9,10-dihydro-9-oxa-10-phosphaphen-anthrene-10-oxide (DOPO), and then DOPO reacts with arylaldehyde to form secondary alcohol. Therefore, bisbiphenylacylphosphine oxide is prepared by using secondary alcohol under oxidation. Also, under a coupling reaction, DOPO reacts with the acid chloride compounds by using a Lewis acid as a catalyst to prepare bisbiphenylacylphosphine oxide.