Fabrication of nanostructured graphene (Gr) for gas sensing applications has become increasingly attractive. For the first time, 3D graphene flowers (GF) cluster patterns are grown directly on an Ni foam substrate by inexpensive homebuilt microwave plasma-enhanced chemical vapor deposition (MPCVD) using the gas mixture H-2/C2H4O2@Ar as a precursor. The interim morphologies of the synthesized GF are investigated and the growth mechanism of the GF film is proposed. The GF are decomposed to few-layer Gr sheets by ultrasonication in ethanol. For the first time, MPCVD-synthesized Gr is exploited to fabricate a gas sensor that exhibits an ultrahigh sensitivity of 133.2 ppm(-1) to NO2. Outstanding sensor responses of 1411% and 101% to 10 ppm and 200 ppb NO2, respectively, are achieved. Furthermore, a low theoretical detection limit of 785 ppt NO2 is achieved. An ultrafast (within 2 s) recovery is observed at room temperature, and an imbedded microheater is employed to improve the selectivity of NO2 detection relative to humidity. This work represents a simple, clean, and efficient route to synthesize large-area cauliflower Gr for gas detection with high performance, including ultrahigh sensitivity, good selectivity, fast recovery, and reversibility.