An optical fingerprint imaging system (200) and an optical assembly (300, 400, 500, 600, 700) are provided. The optical fingerprint imaging system (200) includes: a sensor (203, 302, 402, 502, 602, 702) and a light source (202, 301, 401, 501, 601, 701); wherein the sensor (203, 302, 402, 502, 602, 702) comprises a substrate (2031, 3021, 4021, 5021, 6021, 7021) and a photosensitive layer (2032, 3022, 4022, 5022, 6022, 7022), the substrate (2031, 3021, 4021, 5021, 6021, 7021) has a first surface and a second surface which is opposite to and lower than the first surface, and the photosensitive layer (2032, 3022, 4022, 5022, 6022, 7022) is in contact with the first surface of the substrate (2031, 3021, 4021, 5021, 6021, 7021); and wherein the light source (202, 301, 401, 501, 601, 701) is disposed at a position lower than the first surface and higher than the second surface, and light emitted from the light source (202, 301, 401, 501, 601, 701) is adapted to be guided by the substrate (2031, 3021, 4021, 5021, 6021, 7021) to the first surface of the substrate (2031, 3021, 4021, 5021, 6021, 7021). Accordingly, an optical fingerprint imaging system (200) with a reduced thickness, and a simplified structure is provided.