The invention discloses a concentric pipe layered steam injection system and a method. A cyclone and a phase mixing device are arranged in a steam inlet passage of a steam distributor of the concentric pipe layered steam injection system, the outlet end of the steam inlet passage is respectively connected with a first steam outlet passage and a second steam outlet through a dryness distribution chamber, an inner steam injection flow passage and an outer steam injection flow passage of a well opening device are respectively connected with a steam distributor through an inner steam injection opening and an outer steam injection opening, a concentric type layered steam injection pipe post is connected under the well opening device and realizes the steam injection to a first underground oil layer through the concentric steam distributor arranged on an outer steam injection pipe post, and the steam is injected to a second underground oil layer through an inner steam injection pipe post. The steam dryness from a boiler can be regulated, and in addition, the double-flow-passage layered steam injection is realized, so the accuracy of the steam injection quantity is ensured, simultaneously, certain compensation is provided for the thermal stress deformation generated by the pipe posts through high temperature in the well in the steam injection process, the steam injection effect is improved, and simultaneously, the occurrence of the condition of the pipe post damage is avoided.