A light source device with a replaceable filament comprises a light source and an envelope. The light source includes a filament tube enclosing a filament and electrodes. The filament tube further has an electrode cap and a cross electrode formed at each end thereof. The envelope includes at least one concave reflector, a light outlet, a cross electrode seat correspondingly engaged with the cross electrode of the filament tube and a positioning electrode seat for inserting the filament into the envelope and being correspondingly engaged with the electrode cape of the filament tube. The light source is easily replaced by means of inserting the light source into the envelope through the cross electrode seat. After the light source is installed in the envelope, the filament of light source is approximately aligned with the light outlet of the envelope without calibration.