The invention discloses standard oil for replacing No. IRM903 standard oil for the oil resistance detection of rubber and a preparation method of the standard oil. The standard oil is prepared from the following components in a mixing proportion by weight: 13.5 to 14.5 percent of diisoamyl naphthaline, 25 to 45 percent of KN4010 naphthenic rubber oil, 45 to 65 percent of KN4006 naphthenic rubber oil, 5 to 10 percent of KG-12 rubber oil, 5 to 15 percent of KG-06 rubber oil and 0.1 to 0.5 percent of antioxidant T501. A formula of the standard oil is independently researched and developed, and can be used for replacing the No. IRM 903 standard oil imported from America; further, the adopted raw materials can be all purchased at home; the importation is avoided; the cost is effectively decreased.