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We report a study of ultrafast dynamics of photoexcited electrons and phonons in Sr _3 Ir _4 Sn _13 using dual-color transient reflectivity change ( ${\rm{\Delta }}R/R$ ) measurements. Time resolved optical spectroscopy of collective excitations reveal the marked features near its structural phase transition temperature ${T}^{* }\simeq 147$ K. Two distinctive oscillatory timescales in ${\rm{\Delta }}R/R$ have been clearly resolved. The rapid THz-range oscillations are attributed to the dynamics of the optical phonons which strongly correlate to the structural phase transition. The slow GHz-range oscillatory phenomenon which only occurs below about 150 K is associated with the dynamic response of the longitudinal-acoustic phonons. These low-energy phonons show a softening feature on approaching the transition temperature, also indicating a strong relevance to the structural phase transition. The information that we demonstrated would provide a deeper understanding of the structural phase transition in Sr _3 Ir _4 Sn _13 .