In this paper, a novel photonic analog-to-digital conversion scheme with differential encoding based on vector superposition is proposed and demonstrated. Two pulse sources with different center wavelengths are employed to modulate the input signal by a phase modulator, the modulated signal is sent to a delay-line interferometer to achieve two differential modulated signals with a specific phase difference. By adjusting and combining the intensity of two differential modulated signals with a vector superposition module, the desired phase shifts among different transfer functions can be obtained. Compared with most existing photonic analog-to-digital conversion schemes, the proposed scheme can differentially encode the input signal with improved bit resolution; furthermore, this scheme features its relative simpler configuration, because only one phase modulator, one delay-line interferometer and a vector superposition module are required; in addition, since the desired phase shifts of transfer functions are realized by attenuating the signal intensities, the proposed scheme can effectively alleviate the problem of phase bias-drift induced by modulators. Proof-of-concept experiment of a 4-bit photonic analog-to-digital conversion system based on the proposed scheme is successfully carried out, which demonstrates the feasibility of the approach.