  We present a new empirical fitting method for the optical light curves of
Type Ia supernovae (SNe~Ia). We find that a variant broken-power-law function
provides a good fit, with the simple assumption that the optical emission is
approximately the blackbody emission of the expanding fireball. This function
is mathematically analytic and is derived directly from the photospheric
velocity evolution. When deriving the function, we assume that both the
blackbody temperature and photospheric velocity are constant, but the final
function is able to accommodate these changes during the fitting procedure.
Applying it to the case study of SN~2011fe gives a surprisingly good fit that
can describe the light curves from the first-light time to a few weeks after
peak brightness, as well as over a large range of fluxes ($\sim 5$\, mag, and
even $\sim 7$\,mag in the $g$ band). Since SNe~Ia share similar light-curve
shapes, this fitting method has the potential to fit most other SNe~Ia and
characterize their properties in large statistical samples such as those
already gathered and in the near future as new facilities become available.
