A star-shaped single-polymer system based on 7,7′-(5,5′-(9-(4-(3-hexyl-5-(7-(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)phenyl)-9H-carbazole-3,6-diyl)bis(4-hexylthiophene-5,2-diyl))bis(4-(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole) (TM) as red emissive cores, benzothiadiazole (BT) as green emissive dopants and polyfluorene (PF) as blue arms was designed, synthesized and characterized. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), photophysical, electrochemical, electroluminescence and amplified spontaneous emission (ASE) measurements were performed to unravel the structure–properties relationship of the resulting polymers. By modulating the doping concentration of red, green and blue (RGB) emissive species carefully, high-color-quality saturated white electroluminescence (EL) was achieved with a current efficiency of 2.09 cd A⁻¹, and Commission Internationale d'Eclairage (CIE) coordinates of (0.34, 0.33) for TM-R3G4 containing 0.03 mol% red core and 0.04 mol% green dopants. Particularly, the CIE coordinates matched well with the values of standard saturated white emission (0.33, 0.33). The saturated white EL of the devices could be mainly attributed to the incomplete energy transfer and suppressed intermolecular interactions in the star-shaped single-polymer system. Moreover, TM-R3G4 exhibited a relatively low threshold of 86.8 ± 5 μJ cm⁻², showing intriguing potential as gain media for organic lasers. This study sheds light on understanding the photophysical nature of the resulting star-shaped single-polymer system with simultaneous RGB emission.