The downhole upwelling microsphere measuring instrument needs to realize wireless data transmission function in drilling fluid environment, and its diameter is generally not more than 10mm, which limits the size of wireless transceiver structure. At the same time, MWD tasks during a well drilling process usually require the deployment of thousands of microsphere gauges, and traditional antennas mostly use low-loss plates, which is not conducive to large-scale applications. To solve the above problems, this paper proposes the method of bending the radiation patch and grooving the ground plane to improve the traditional PIFA antenna. The plane size of the antenna is only 17 × 8.4 × 0.049mm, which is 68.3% smaller than that of similar antenna designs. The antenna is manufactured by flexible printed circuit technology, bent at 175° and mounted on the microsphere measuring instrument surface. The resonant point frequency measured by the antenna in the drilling fluid with relative dielectric constant of 81 and conductivity of 1.258S /m is 310MHz, corresponding return loss is -32.4dB, and the bandwidth of -20dB is 35MHz. The antenna has ideal omnidirectional radiation characteristics in drilling fluids with different relative dielectric constant and conductivity. Compared with the antennas proposed in other literatures, the antennas proposed in this paper are advanced in dimensions, resonant frequency and return loss. In summary, the antenna proposed in this paper can be used for wireless communication of the microsphere measuring instrument in the drilling fluid environment, which changes the data transmission mode of the original microsphere measuring instrument from wired communication to wireless communication, reduces the difficulty of designing the downhole measuring sub, and improves the timeliness of data transmission.