The pedestrian was airlifted to hospital after the incident on the A4811 New Road, in Newtown, on Thursday.
A 27-year-old man has been arrested on suspicion of driving a vehicle dangerously and driving while unfit through drugs, and released under investigation.
A police spokesman said: “Dyfed-Powys Police attended a road traffic collision which occurred at around 8.40am,Thursday, 12th May 2022 on the A4811 New Road, Newtown.
"A trailer, being towed by a DAF lorry, and a pedestrian were involved in the collision. The pedestrian was taken to hospital by air ambulance.
"The road was closed, with diversions in place.
"A 27-year-old man was arrested on suspicion of driving a vehicle dangerously and driving whilst unfit through drugs. He has been released under investigation pending further police enquiries.
"Anyone who witnessed the incident or anyone with information that could help officers with their investigation is asked to report it to Dyfed-Powys Police, either online at: https://bit.ly/DPPContactOnline, by emailing email@example.com, or by calling 101. If you are deaf, hard of hearing, or speech impaired text the non-emergency number on 07811 311 908. Quote reference: DP-20220512-044.”