The present invention provides a light source fixing device for fixing a light emitting element. The light emitting element comprises a light source and a supporting plate for supporting the light source. The light source fixing device comprises a heat dissipating support which comprises a receiving recess. The receiving recess is adapted to receive the supporting plate. The light source fixing device further comprises an elastic element disposed in the receiving recess, and the elastic element is adapted to apply an elastic force on the supporting plate so that the supporting plate is fixed in the receiving recess. The present invention further provides a light source assembly and an assembling method thereof. The light source assembly features a simple assembling process, high reliability and a low cost.