Recently, the LHCb Collaboration observed two new bottom baryons, the Ξ b (6227) − and the Σ b (6097) ± , which arouses considerable interest in their inner structures. Stimulated by this, we re-examine the low-lying spectrum of the singly heavy baryons in the heavy quark-diquark picture. Our computation indicates that a linear Regge relation, which is derived from the rotating string model, is sufficient to describe the mass spectrum of all observed singly heavy baryons, predicting that the quantum numbers of the charmed baryons Σ c (2800) and the charm-strange baryon Ξ ′ c (2930), which remain unknown experimentally, are preferably J P = 3/2 − . Similar calculations for the bottom partners support that the newly observed baryons Σ b (6097) − and Ξ ′ b (6227) − are the 1P-wave excitations of the baryons Σ b and Ξ ′ b with J P = 3/2 − . PACS numbers: 14.40.Be, 12.40.Nn, 12.39.Ki ′ b (6227) − . Finally, the paper ends with the discussions and summary in Sec. V. II. SPIN-INDEPENDENT MASSES FOR SINGLY HEAVY BARYONSThe notion of Regge trajectory or Regge mass relation for hadrons, which connects high energy scattering and the spectrum of hadrons in general theory [14], provides us a powerful way to organize the whole spectrum of a given family of the hadrons. In the case of light hadrons, the Regge trajectories are commonly believed to be linear and parallel, and these features are also tested to be true roughly for the SH hadrons in Ref. [24] provided that the hadron mass undergoes a shift M → M − M Q , with M Q the heavy quark mass. As such, one can estimate the spin-averaged masses of Σ c and Ξ ′ c in their P-wave states with the help of the trajectory information of the partner baryons, Λ c and Ξ 0 c , which has the rich data experimentally, and so forth.