代码至少存在以下问题：

未初始化的g

float g;
 for (k = 0; k < alpha; k++)
   g += (double) (-1 / beta) * log(1 - ((double) rand() / RAND_MAX));

一个月一个月

log(1-((double)rand()/RAND_MAX))可能会提供log(0)。我怀疑log(1 - (((double) rand() + 0.5)/ (RAND_MAX + 1u)))会提供更好的分发。

3个FP类型

float, double, long double有很多上变频和下变频。
始终使用double，直到确定需要其他宽度。

未使用的对象

不使用float msea=0.0,mseb=0.0。提示：启用所有警告并保存时间。

使用%g

"%g"的信息量更大。

// printf("MSE of alpha is = %Lf", msea);
printf("MSE of alpha is = %Lg", msea);

OP报告“我做了更改，它仍然在输出“。

MSE of alpha is = 105.474
MSE of beta is = 31.4536

我怀疑OP还没有做出所有的改变。

# include <stdio.h>
# include <math.h>
# include <stdlib.h> // Don't forget to include this

int main() {
  int n, NN; // Sample size and number of simulations
  double alpha = 3, beta, suma = 0.0, sumb = 0.0, msea, mseb;
  int i, j, k;
  printf("Enter the number of simulations");
//  scanf("%d", &NN);
  printf("Enter the sample size");
//  scanf("%d", &n);
  printf("Enter the value of beta");
//  scanf("%f", &beta);
  NN = 1000;
  n = 20;
  beta = 1.5;
  //Simulation starts to calculate MSE
  for (i = 0; i < NN; i++) {
    double sum = 0.0, sumsq = 0.0; //Vlaue is reset at every iteration, so declared inside i loop
    for (j = 0; j < n; j++) //each sample
        {
      double g = 0;
      for (k = 0; k < alpha; k++)
        g += (double) (-1 / beta)
            * log(1 - (((double) rand() + 0.5) / (RAND_MAX + 1u)));
      sum += g;
      sumsq += g * g;
    }
    double xbar = sum / n;
    double var = sumsq / n - xbar * xbar;
    suma += pow((xbar * xbar / var - alpha), 2);
    sumb += pow((xbar / var - beta), 2);
  }
  msea = suma / n;
  mseb = sumb / n;
  printf("MSE of alpha is = %g\n", msea);
  printf("MSE of beta is = %g\n", mseb);

  return 0;
}