The synthesis of large-scale monolayer single-crystal MX2 (M=Mo, W; X=S, Se), a typical transition steel dichalcogenides (TMDs), is the premise for his or her future purposes. In contrast with insulating substrates akin to SiO2 and sapphire, Au is extra beneficial for the quick development of TMDs by chemical vapor deposition (CVD). Lately, large-scale single-crystal WX2 was efficiently grown and transferred on Au. In sharp distinction, the expansion and switch for monolayer MoX2 continues to be very difficult, as a result of Au has a better solubility of Mo and stronger interplay with MoX2 than WX2. In contrast with essentially the most studied MoS2, MoSe2 is superior in lots of points due to the narrower band hole and tunable excitonic charging results. Nonetheless, the synthesis of large-scale single-crystal MoSe2 on Au has not been reported to this point. Right here, a pre-alloying CVD methodology was developed to resolve the issues for the expansion and non-destructive switch of MoX2. It has realized the ultrafast development (30 s) of submillimeter-scale (560 µm) single-crystal MoSe2 for the primary time. As-grown samples are strictly monolayers with excessive optical and electrical properties, which could be simply transferred with out sacrificing Au foils by electrochemical effervescent methodology. It was discovered that pre-alloying not solely passivates the energetically lively websites on Au, but in addition weakens the interplay between Au and MoSe2, which is accountable for the ultrafast development and simple switch of MoSe2. This methodology can also be common for the quick development and non-destructive switch of different 2D TMDs.