The emission properties of double-pulse (DP) over single-pulse (SP) femtosecond laser breakdown spectroscopy (fs- LIBS) of polymethyl methacrylate (PMMA) were investigated. The signal enhancements in the DP fs-LIBS strongly depended on the DP delay and were influenced by the type of emission particles. Intensity enhancement of emission lines increased in the sequence of molecules, neutral atoms, and ions. Electron density and temperature were reported to characterize the plasmas. Both the electron density and plasma temperature exhibit similar variation trajectories with respect to the DP delay and feature a distinct increase at an optimal DP delay of ~80 ps, indicating reheating of preproduced plume is responsible for the emission enhancement. The dependence of the signal emission on laser energy was also studied, showing the emission intensity was linear to the pulse energy. However, the signal enhancement was nonlinear to the pulse energy, suggesting that the signal enhancement was related to the energy coupling efficiency of second pulse to the first pulse generated plume.