The mini-outbursts of the black hole X-ray transients are a kind of outburst with low peak luminosity and short duration. Due to the lack of observational data, the physical mechanism is still unclear. Using data from RXTE (Rossi X-Ray Timing Explore) satellite from January 28 to March 14, 2001, we studied the X-ray energy spectrum characteristics in the mini-outburst from X-ray transient XTE J1550--564 of the black hole. It was found that the X-ray energy spectrum of XTE J1550--564 in this mini-outburst could be well fitted with a power law spectrum. The evolution of the hardness-intensity diagram throughout the outburst is not a canonical ``q-track pattern', but lies on the far right. In addition, we also analyzed the correlation between X-ray photon index T and unabsorbed 2- 10 keV X-ray fux F2- 10keV, and found that there is an anti-correlation between T and F2 10keV. Meanwhile, the photon index T is 1.35 to 1.72. Our results suggest that the outburst in 2001 has been in a low/hard state, its X-ray radiation mainly comes from the low-efficient accretion mode, such as ADAF (Advection-Dominated Accretion Flow).