We analyse the multiwavelength photometric and spectroscopic data of 12 ultraluminous infrared galaxies (ULIRGs) at z∼ 1 and compare them with models of stars and dust in order to study the extinction law and star formation in young infrared (IR) galaxies. Five extinction curves, namely the Milky Way (MW), the pseudo-MW which is MW-like without the 2175 Å feature, the Calzetti and two supernova (SN) dust curves are applied to the data by combining with various dust distributions, namely the uniform dust screen, the clumpy dust screen, the internal dust geometry, and the composite geometry with a combination of dust screen and internal dust. Employing a minimum χ2 method, we find that the foreground dust screen geometry, especially combined with the 8–40 M⊙ SN extinction curve, provides a good approximation to the real dust geometry, whereas internal dust is only significant in two galaxies. The SN extinction curves, which are flatter than the others, reproduce the data of eight (67 per cent) galaxies better. Dust masses are estimated to be in excess of ∼108 M⊙. The inferred ages of the galaxies are small, eight of which range from 10 to 650 Myr. The SN-origin dust is the most plausible to account for the vast amount of dust masses and the flat slope of the observed extinction law. The inferred dust mass per SN ranges from 0.01 to 0.4 M⊙ per SN.