An analog-to-digital converting system includes multiple stages of analog-to-digital converters (ADCs) and a skew calibration circuit. The multiple stages of ADCs are configured to sample a test signal according to multiple interleaved clock signals, respectively, so as to respectively generate multiple stages of quantized outputs. The analog-to-digital converting system has a sampling frequency resulting from operations of the multiple stages of ADCs. The test signal has a first frequency and the sampling frequency is N times the first frequency, and N is an odd number larger than 1. The skew calibration circuit is configured to sequentially analysis, for every N stages, the multiple stages of quantized outputs to generate multiple digital codes. The skew calibration circuit is further configured to calibrate a time skew of the analog-to-digital converting system according to a comparison result between the multiple digital codes and a reference code.