We investigated the influence of MnO2 doping on the electric properties and thermal stability of (Ca-0.4,Sr-0.6)Bi4Ti4O15 high-temperature piezoelectric ceramics. The XRD patterns reveal that Curie temperature (T (C)) of the CSBTM-x ceramics slightly changes with increasing MnO2 content due to the slightly distorted lattice. The temperature dependence of inverse dielectric constant (1/epsilon (r)) reveals that the ceramics undergo a first-order phase transition. MnO2 doping with proper content (1, 2, 3 mol%) can significantly lower dielectric loss (tan delta) and enhance piezoelectric coefficient (d (33)) of the CSBTM-x ceramics. The dependence of annealing temperature on d (33) indicates that the thermal stability becomes lower with increasing MnO2 content (x), and the CSBTM-x ceramics doped with MnO2 at low concentrations (x is less than 4) have good thermal stability. Optimal piezoelectric properties of the ceramics with 4 mol% MnO2 additive (CSBTM-4) are obtained: d (33) = 23.8 pC/N, k (p) = 0.075, Q (m) = 3221, and T (C) = 626 A degrees C. The CSBTM-2 ceramics have excellent thermal stability with good dielectric and piezoelectric properties at 300 A degrees C: d (33) = 20.8 pC/N, (d (33T) - d (33RT) )/d (33RT) = -0.95 %, and tan delta = 0.00475 at 1 MHz. The dielectric and piezoelectric properties as a function of MnO2 content (x) indicate that the introduction of Mn ions into the CSBTM-x ceramics produces both "soft" and "hard" doping effects. The d (33) of CSBTM-0, 2, 4, and 8 ceramics slightly decreases (less than 2 %) when the annealing temperature is below about half of the T (C) (300 A degrees C), which is in accordance with the fact that the operating temperature of piezoelectric ceramics in sensors or detectors seldom exceeds about half of T (C) of the ceramics.