1. A dual-phase virtual metrology method, comprising: obtaining a plurality of sets of historical process data belonging to a production equipment; obtaining a plurality of historical measurement values from a measurement equipment, wherein said historical measurement values are the measurement values of the workpieces which are manufactured in accordance with said sets of historical process data respectively; establishing a first conjecture model by using said sets of historical process data and said historical measurement values, wherein said first conjecture model is built in accordance with a conjecture algorithm; waiting for collecting the process data of a plurality of workpieces from said production equipment; After the collection of the process data of one of said workpieces from said production equipment is completed, promptly performing a phase-one conjecture step, wherein said phase-one conjecture step comprises; Inputting the process data of said one of said workpieces into said first conjecture model, thereby computing a phase-one virtual metrology value (VM I ); and when the actual measurement value of a selected workpiece of said workpieces is obtained from said measurement equipment, performing a phase-two conjecture step, wherein said phase-two conjecture step comprises: retraining said first conjecture model by adding the set of the process data and actual measurement value of said selected workpiece to said sets of historical process data and said historical measurement values; or tuning said first conjecture model by using the set of the process data and actual measurement value of said selected workpiece, thereby turning said first conjecture model into a second conjecture model; inputting the process data of all the workpieces in a cassette belonging to said selected workpiece into said second conjecture model, thereby re-computing a phase-two virtual metrology value (VM II ) of each workpiece in said cassette; and updating said first conjecture model with said second conjecture model for computing the phase-one virtual metrology value of the workpiece subsequently manufactured by said production equipment.