A musical instrument stand assembly with a foldable pedal includes a main stand body, and the main stand body includes a main rod, and the bottom of the main rod includes two support rods each with a recession, and the recession includes a first circular serration; two side frames, and a sidewall of each side frame includes a protrusion and a second circular serration at the protrusion, and each side frame is installed by engaging each protrusion with the recession of each support rod, such that the first and second circular serrations are engaged with each other. With the design of rotably engaging the two side frames and the two support rods, the main stand body of the musical instrument has the function of adjusting its inclination with respect to a vertical position, and the effect of conveniently folding the pedal of the musical instrument.