Abstract: The dome-roof consisted of fan-shaped plates is the structure which is generally adopted in domastic construction for dome-roof tank. However, the problem that insufficient width of lap joint between the middle parts of fan-shaped plates often exists due to the plate with the shape of straight lines at its two sides and arcs at its two ends designed according to its stretched-out view by usual practice, resulting in reconstruction of the dome roof, longer construction period and decreasement of economical benefits. Through profound study on this problem starting with analyzing the geometrical equations calculating fan-shaped plate given in current literatures, the cause that the insufficient width of lap joint between the fan-shaped plates is found, and it is concluded that the sides of fan-shaped plate are not straight lines but curves by geometrical calculating for fan-shaped plate with butt joint (not lap joint Structure. New relative formulae are given, involving fewer relative parameters. It is more simple to calculate the arc length of the two ends of the plate and is easier to observe the curves' shape.