The present invention provides an optical element and an illuminating device. The optical element includes: an incident surface; and an emitting section formed on an opposite side to the incident surface. The emitting section includes a plurality of protrusions. When each of the plurality of protrusions is projected onto a reference plane perpendicular to the incident surface to form a reference contour, the reference contour is substantially a trapezoid. A height of the reference contour and a height of a triangle formed by extending two legs of the reference contour satisfy a predefined condition.