Deformation and vibration of thin-wall workpieces during machining could induce residual stresses in the final products, which makes real-time monitoring of the strain field critical to optimizing machining process and minimizing workpiece stresses. However, traditional methods based on strain-gauge measurements are inapplicable because materials are removed during machining. Illustrated in the context of duplex lathe-turning that offers an effective means to reduce some unbalanced cutting forces, this paper presents a non-contact sensing method to reconstruct the continuous strain fields of a thin-wall workpiece under external time-varying cutting and clamping loads. This proposed method based on the linear combination of mode shapes obtained offline for online updating has been numerically verified against finite element analysis; both static and dynamic conditions are considered. Experiment results confirm that the method is effective in capturing the dynamic behaviors in practice.