The ternary compound aluminum gallium arsenide is an important material that can be used in all-optical solid-state ultrafast diagnostic technology. The low-temperature-epitaxially-grown AlGaAs (LT-AlGaAs) not only has the characteristics of ultra-short carrier lifetime of low-temperature-grown gallium arsenide (LT-GaAs), but also possesses the advantage of adjustability of band gap, which will provide great flexibility for the design of ultra-fast diagnostic systems. We use low-temperature epitaxial growth technology to grow AlGaAs on a GaAs substrate. The low-temperature-grown AlGaAs can effectively absorb 400 nm pump light to generate excess carrier. Therefore, we use a femtosecond laser with a wavelength of 800 nm and a pulse width of 200 fs as a light source to generate 400-nm pump light after passing through the BBO crystal, and 800 nm light without frequency doubling as the probe light. Using such a light source, we build a pump probe experimental platform to test the LT-AlGaAs. We normalize the experimental results and deconvolute it with the normalized laser pulses to obtain the response function of the semiconductor to the pump light. Therefore, we know that the nonequilibrium carrier relaxation time is less than 300 fs, and the nonequilibrium carrier recombination time is 2.08 ps. Due to the special passivation process, the effect of surface recombination on the carrier decay process is greatly reduced. The As clusters introduced by low-temperature epitaxial growth form deep level defects are the main factor for accelerating carrier recombination. In order to understand the complex process of photogenerated nonequilibrium carriers in depth, we use the indirect recombination theory of single recombination center to calculate the carrier recombination process, and establish an LT-AlGaAs carrier evolution model. Thus we obtain the key physical parameter related to the recombination rate, which is the carrier trapping area. We also use a theoretical model of carrier-regulated refractive index to calculate the effect of carrier concentration on the amount of change in refractive index. Combining our AlGaAs carrier evolution model, we simulate the refractive index change process of LT-AlGaAs after being illuminated by pump light. The simulation results are in good agreement with the experimental results. The method can be used for the quantitative analysis of carrier evolution characteristics of semiconductor materials, and it can conduce to the optimization and improvement of ultra-fast response semiconductor materials.