The slow light technology of the rectangle signal propagating in erbium-doped fiber (EDF) has potential applications in the fields of all optical communication and optical fiber sensing. The method of using harmonics fractional delay to evaluate the slow/fast light of rectangle signal propagating in the EDF is proposed, and the characteristics of phase delay for fundamental and high order harmonics components are analyzed for the first time based on the rate equations and the theory of the coherent population oscillations (CPO). We experimentally demonstrate the dependences of fundamental fractional delay on input power and optical gain. The maximum fractional delay 20% is obtained when the input power is about 8 mW without pump. The negative fractional delay-20% is also achieved and it will increase with the rising of the optical gain. The Nth-order fractional delays (N=1, 3, 5, 7) of rectangle signal propagating in EDF without pump are investigated. Their maximum fractional delays are all about 0.07 and the corresponding fundamental modulation frequencies are 22, 7, 5 and 3 Hz, respectively. What is more, the Nth-order fractional delays (N=1, 3, 5, 7) with pump are also investigated. Their maximum fractional delays are all about-0.135 and the corresponding fundamental modulation frequencies are 175, 58, 35 and 25 Hz, respectively. The experiments indicate that the maximum Nth-order fractional delays are equal and they will be achieved at the frequency f/N (the fundamental harmonic fractional delay is maximum at the modulation frequency f). The results show good agreement with CPO and the frequency is also located in the spectral burning hole.