In this paper, we discuss briefly some work on linear measurement in which the liquid level is obtained by means of radioisotope and put forward a formula of activity distribution of radioactive source that strictly satisfies the requirement of linear graduation. The formula is applicable to designing the linear measurement of liquid level or a place where two fluid media meet, calculating activity distribution of the uneven stick radiator or the uneven point radiators group. In addition, we recommend the application of the formula and the methods of computing and examining the activity distribution of radioactive Source, and analyse briefly the graduation of the type LB3121 γ-liquid level gauge, whose activity distribution of radioactive source is found to satisfy the requirement of linear graduation not so strictly.