We study the relationship between the null geodesics and thermodynamic phase transition for the charged AdS black hole. In the reduced parameter space, we find that there exist non-monotonic behaviors of the photon sphere radius and the minimum impact parameter for the pressure below its critical value. The study also shows that the changes of the photon sphere radius and the minimum impact parameter can serve as order parameters for the small-large black hole phase transition. In particular, these changes have an universal exponent of 1 2 near the critical point for any dimension d of spacetime. These results imply that there may exist universal critical behavior of gravity near the thermodynamic critical point of the black hole system. PACS numbers: 04.70.Dy, 04.50.Gh, 04.25.-g I. INTRODUCTIONRecently, there is an extensive study on the black hole thermodynamics in the extended phase space with the cosmological constant being treated as pressure [1][2][3][4][5][6][7]. Interestingly, Kubiznak and Mann observed the thermodynamic stable small-large black hole phase transition for the four-dimensional charged AdS black holes [8], which mimics qualitatively the behavior of the Van der Waals (vdW) fluid. Such small-large black hole phase transition can be traced back to Refs. [9][10][11][12]. Shortly thereafter, the study was generalized to other black hole systems, and some abundant phase structures were discovered, such as the reentrant phase transition, triple point, isolated critical point, and superfluid black hole phase [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] (for a recent review see [28] and references therein).Probing the small-large black hole phase transition is an interesting subject. In Ref.[29], the authors found that there is a dramatic change in the slope of the quasinormal modes (QNMs) before and after the black hole's thermodynamic phase transition. This provides us a dynamic probe for the phase transition. The study was also extended to other AdS black holes [30][31][32][33][34]. These results imply that at low temperature or pressure, the phase transition can be effectively probed by measuring the change of the slope of the QNM frequencies. While at high temperature or pressure, extra measurement should be introduced, such as the non-monotonic behaviors of the imaginary part of the QNM frequencies [34].On the other hand, we would like to study whether there exist some relationships between the gravity and thermodynamics for the black hole system. In Refs. [35,36], the authors pointed out that the presence of photon orbits signals a possible York-Hawking-Page type phase transition. Using the stable photon orbit, the effect of a negative cosmological constant on the instabilities of black hole spacetimes was investigated in Ref. [37].From the side of gravity, the null geodesics plays a center and important role in the black hole physics. In particularly, the photon sphere has a close relation with the strong gravitational effects near the black hole, such as the lensing, shadow, ...