An optical signal transmitting device includes a substrate, light emitting modules, an optical coupling element, an optical fiber module, and a pressing pole. The substrate has a first loading surface and a second loading surface. The optical coupling element is positioned on the first loading surface and includes a first cladding portion and coupling lenses. Each coupling lens has a first sloped surface and a second sloped surface. The light emitting modules are positioned on the second loading surface and spatially correspond to the respective first sloped surfaces. The optical fiber module is positioned on the first loading surface and includes a second cladding portion and fiber cores. Each fiber core has a bare end. The pressing pole presses each bare end to the corresponding second sloped surface. The refractive indexes of the substrate, the coupling lenses, the fiber cores and the air are n1, n2, n3, n0, wherein n3>n2>>n1>n0.