Experimental photoelectron spectrum of Si2C2- was assigned by ab initio Monte Carlo simulation with the multicanonical/histogram reweighting technique. At first, two structures, linear and ring, were optimized at the MP2/6-31G* level. The multireference configuration interaction (MRCI) calculation reveals that the spectrum cannot be assigned by considering only a single isomer. Also, MRCI calculations cannot explain different widths of the peaks in the spectrum. Multicanonical Monte Carlo simulation with configuration interaction calculation at each step of the simulation was performed for both ring and linear isomers to include the effects of large nuclear motion at finite temperature on the spectrum. It was found that a mixture of two isomers at the experimental condition is necessary to assign the spectrum.