Optical biosensors with a high sensitivity and a low detection limit play a highly significant role in extensive scenarios related to our daily life. Combined with a specific numerical simulation based on the transfer matrix and resonance condition, the idea of novel single-waveguide-based microresonators with a double-spiral-racetrack (DSR) shape is proposed and their geometry optimizations and sensing characteristics are also investigated based on the Vernier effect. The devices show good sensing performances, such as a high quality factor of 1.23×105, a wide wavelength range of over 120 nm, a high extinction ratio (ER) over 62.1 dB, a high sensitivity of 698.5 nm/RIU, and a low detection limit of 1.8×10−5. Furthermore, single-waveguide-based resonators can also be built by cascading two DSR structures in series, called twin-DSRs, and the results show that the sensing properties are enhanced in terms of quasi free spectral range (FSR) and ER due to the double Vernier effect. Excellent features indicate that our novel single-waveguide-based resonators have the potential for future compact and highly integrated biosensors.