Triboelectric nanogenerator (TENG) sensors have been widely developed for strain detection in health monitoring. However, current TENG sensors are insufficient to accurately detect the weak strain (< 1%) at the cellular scale (e.g. contraction of cardiomyocytes). Here, we propose a TENG-based strain sensor with a high signal-to-noise ratio (SNR) and stability to quantitatively measure the contractility of cardiomyocytes. 100,000 contact-separation cycles could be stably achieved under the strain of 16.9% (pneumatic pressure = 11 kPa), due to the effective separation of two friction layers between the micro-scale gap (8 µm). 50 µm-thickness friction layer with nano-cracks (average width = 48.1 ± 11.9 nm) lowered the strain detection limit to 0.025% with SNR of 12.1. During 11-day continuous monitoring of cardiomyocytes and drug treatment testings, the device demonstrated high SNR and stability to record the dynamic change caused by contractility of the cardiomyocytes. The flexible strain sensor based on TENG shows the potential of in cardiac pharmacological evaluation strategy.