To meet the demand for high-precision testing of total temperature in high-temperature, high-speed airflows within aeroengines, this paper employs fluid-structure interaction numerical simulation methods to systematically investigate the flow heat transfer and temperature measurement error characteristics of a shielded thermocouple under seven velocity conditions ranging from 0.2 to 0.8 Ma and six temperature conditions from 700℃ to 1 200℃. Results indicate that the shielding cover exhibits significant stagnation effects, reducing flow velocity at the measurement tip by over 80%. Error analysis reveals that all error terms increase with rising Mach number, with growth rates following the sequence: radiation error > thermal conduction error > velocity error. At low Mach numbers (Ma≤0.3), thermal conduction error dominates. However, as Mach number increases, radiation error becomes significantly more influential and emerges as the primary error source. Elevated temperatures further exacerbate the impact of radiation error. Overall, thermal conduction and radiation errors collectively account for over 93% of the total error, constituting the key factors affecting measurement accuracy. After applying an empirical radiation error correction formula, the steady-state error decreased from 17.97 K to 0.64 K, and the overall temperature recovery coefficient improved to above 0.92, significantly enhancing measurement precision.