The present invention discloses a planar optical waveguide device and a manufacture method thereof. The method includes the following steps that: a substrate is provided; a support layer is formed on the substrate; a waveguide layer is formed on the support layer; the waveguide layer is divided into a plurality of strip-shaped waveguides, at least a part of the surface of each strip-shaped waveguide is in sharp-corner transition; and the sharp-corner transition is changed to rounded transition through a reflow technique. According to the planar optical waveguide device and the manufacture method thereof of the invention, the rounded-transitionally waveguides are formed through the reflow technique; since the cross section of each rounded-transitionally waveguide has no sharp corners, and insensitivity to polarization can be realized; since the outer surface of each rounded-transitionally waveguide is a continuous arc surface, loss can be can greatly reduced; and compared with a linear waveguide, the contact area of the rounded-transitionally waveguides and the support layer is small, regions of each rounded-transitionally waveguide which do not contact with the support layer, directly contact with the air, and therefore, the refractive index difference value of the core region and cladding of the waveguide is larger, so that a larger effective refractive index can be obtained, and thus, that the size of the entire device can be further reduced.