The discovery of multiple resonance thermally activated delayed fluorescence (MR-TADF) materials with remarkable narrowband emission has opened a new avenue for the development of organic light-emitting diodes (OLEDs) with high color purity. However, the lack of construction strategies for purely red MR-TADF materials significantly impedes their application in full-color high-definition displays. Herein, we propose a unique and handy approach of spiro-carbon-locking and sulfur-embedding strategy to modify the parent MR-TADF framework, resulting in a red MR-TADF emitter with high color purity. The reported MR-TADF molecule (namely, FSBN) demonstrates a pure red emission with an emission maximum of 621 nm in toluene solution. The OLED with FSBN as emitter exhibits Commission Internationale de l'Éclairage (CIE) coordinates of (0.67, 0.33), which exactly matches the red standard defined by the National Television Standards Committee (NTSC). Importantly, the single-host OLED achieves a high power efficiency (PE) of up to 50.1 lm W