This　paper　proposes　a　new　reader　solenoid　coil　with　a　metal　tube　for　the　use　in　high-temperature　water,　mud,　and　other　impurities　in　an　oil　well　downhole.　The　coil　can　read　commands　transmitted　from　the　ground　surface　via　inactive　microtags　and　write　the　sensor　data　collected　in　the　downhole　into　the　microtags　for　transmission　to　the　ground.　By　combining　transformer　theory　and　electromagnetic　field　theory,　the　main　parameters　of　the　reader　solenoid　coil　in　the　harsh　industrial　environment　in　a　downhole　are　calculated,　and　the　main　factors　responsible　for　radio-frequency　identification　communication　failure　at　125　10kHz　of　the　reader　solenoid　coil　are　analyzed.　Then,　a　mathematical　model　is　developed　to　describe　the　relationship　among　the　electromagnetic　field　density,　the　ac　resistance　of　complex　permittivity,　and　the　complex　permeability　with　water　and　the　metal　tube　of　the　reader　solenoid　coil.　Methods　to　reduce　the　changes　in　the　inductance　and　resistance　are　presented.　The　minimum　flux　density　of　the　reader　solenoid　coil　is　designed　to　provide　a　continuous　energy　supply　to　inactive　microtags,　and　the　identification　rate　by　the　reader　is　100　at　125　10kHz.　Furthermore,　the　reader　solenoid　coil　was　tested　in　an　industrial　environment　in　the　downholes　of　six　oil　well　drilling　systems　for　two　years.